Citation: Shu, W.; Ming, A.; Zhang, J.;

Li, H.; Min, H.; Ma, J.; Yang, K.; Li, Z.;

Zeng, J.; Wei, J.; et al. Effects of

Close-to-Nature Transformation on

Soil Enzyme Activities and Organic

Carbon Fractions in Cuninghamia

lanceolata and Pinus massoniana

Plantations. Forests 2022, 13, 872.

https://doi.org/10.3390/f13060872

Academic Editor: Choonsig Kim

Received: 22 March 2022

Accepted: 30 May 2022

Published: 2 June 2022

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Article

Effects of Close-to-Nature Transformation on Soil EnzymeActivities and Organic Carbon Fractions inCuninghamia lanceolata and Pinus massoniana PlantationsWeiwei Shu 1,2 , Angang Ming 1,2,*, Jihui Zhang 3, Hua Li 1,2, Huilin Min 1,2, Junxu Ma 1,2, Kun Yang 1,2,Zhongguo Li 1, Ji Zeng 1, Juling Wei 1, Zhaoying Li 1 and Yi Tao 1

1 Experimental Center of Tropical Forestry, Chinese Academy of Forestry, Pingxiang 532600, China;shuweiwei_@outlook.com (W.S.); lihua782003@sina.com (H.L.); minhuilin828@163.com (H.M.);majunxu1994@163.com (J.M.); yk4225@163.com (K.Y.); zhongguo1980@126.com (Z.L.);zengji_2006@sina.com (J.Z.); weijuling@126.com (J.W.); lzying1877@sina.com (Z.L.); tyrzjl@126.com (Y.T.)

2 Guangxi Youyiguan Forest Ecosystem Research Station, Pingxiang 532600, China3 Guangxi Forest Inventory and Planning Institute, Nanning 530000, China; zhangjihui@caf.ac.can* Correspondence: mingangang0111@163.com; Tel.: +86-0771-8585023; Fax: +86-0771-8526320

Abstract: Soil enzymes are involved in the process of mineralization of soil organic matters. Theclose-to-nature transformation (CNT) of plantations changes the soil enzyme activities by changingthe composition of stand vegetation, which in turn affects the change process of soil organic carbon.We therefore selected two typical coniferous plantations in southwest China, Pinus massoniana andCunninghamia lanceolate, to explore the effects of CNT on soil enzyme activities and soil organiccarbon, and the relationship between them is explored through comparative study. Compared withcontrol stands (CCK and PCK), CNT enhanced soil organic carbon; the content of water-solubleorganic carbon in the 0–10 cm soil layer of the transformed C. lanceolata plantations (CCN) is 81.29%higher than those in the control stands (p < 0.05); the contents of particulate organic carbon andwater-soluble organic carbon in the 10–30 cm soil layer are 95.42% and 48.68% higher than those inthe control stands (p < 0.05), respectively; after the CNT, the protease, urease, and acid phosphatase inC. lanceolata plantations were higher than control stands, while protease and catalase in P. massonianaplantations were higher than control stands. Correlation analysis showed that catalase and proteasewere more closely related to organic carbon components than other organic enzymes; redundancyanalysis (RDA) results show that pH and total nitrogen are key factors that cause changes in carbonfractions after the CNT. In general, CNT enhanced soil organic carbon in coniferous plantations,which was more conducive to soil organic carbon accumulation but had a negative effect on soilorganic carbon stability to a certain extent. Therefore, the effect of tree species configuration on soilcarbon stability components should be considered in the forest management practice.

Keywords: close-to-nature transformation; enzyme activities; organic carbon fractions

1. Introduction

The increase in forest carbon sink is an important way to achieve “peak emissions andcarbon neutrality”. The forest soil is the main organic carbon pool in the terrestrial carboncycle. The changes in its carbon storage play an important role in regulating the globalcarbon balance and mitigating the greenhouse effect [1]. The area of plantations in Chinaranks first in the world, with an area of about 79.5428 million hm2, which has a huge carbonsink capability [2]. It is considered to be the possible mechanism and the most potentialchoice for mitigating global change. It plays an increasingly significant role in mitigatingclimate change and promoting the realization of “peak emissions and carbon neutrality” [3].Therefore, how to increase the storage and stability of soil carbon in plantations has becomea hot topic and an urgent problem to be solved for scientists at home and abroad.

Forests 2022, 13, 872. https://doi.org/10.3390/f13060872 https://www.mdpi.com/journal/forests

Forests 2022, 13, 872 2 of 13

Soil organic carbon (SOC) is an important part of forest soil carbon pool and animportant indicator of forest soil fertility [4]. However, due to the high background valueof soil organic carbon, the response to changes in the short-term management model showsa certain lag, which cannot accurately reflect the change in soil quality and transformationrate in a short time [5]. It is found in the study that the active organic carbon of soilis more sensitive to forest management measures than total organic carbon, and can betaken as an indicator of early changes in organic carbon, which are usually represented bywater-soluble organic carbon, readily oxidation organic carbon, particulate organic carbon,etc. The inactive organic carbon represents the accumulation and sequestration of soilorganic carbon, such as recalcitrant organic carbon and recombinant organic carbon [6,7].Therefore, the soil carbon pool fractions are used to quantitatively analyze the changes insoil organic carbon in south subtropical plantations, which is more conducive to studyingthe changes in soil organic carbon pool in this region.

“Close-to-nature transformation (CNT)” is a forest management model that is inclusiveof forestry production and ecological protection, which originated in Germany. It is afeasible way to achieve multi-objective management of plantations [8]. The long-termpractice has proved that this model can achieve different harvest objectives under thepremise of stable development of forest ecosystems so as to maximize the comprehensivebenefits of forests [9]. Compared with the monoculture plantations, the CNT has changedthe vegetation community, tree species composition, quantity, and quality of litterfall,which affects the soil microhabitat, resulting in changes in soil nutrients, stand productivity,and species diversity [10,11]. The reduction in soil carbon input and the increase in carbonemissions in the early stage of CNT led to the loss of soil organic carbon and certain nutrientelements [12]. However, CNT was able to improve the species diversity of plantations,increase the carbon storage, improve the physical and chemical properties of soil, andreduce the greenhouse gas emissions [13], indicating that the impact of CNT on soil carbonwas more complicated.

In addition, soil enzyme activity greatly affected the decomposition, transformation,and storage of soil organic carbon in the process of soil carbon cycling [14]. Amongthem, urease and acid phosphatase indirectly affect the soil carbon cycle by affectingsoil nitrogen and phosphorus content [15], and catalase is also related to the conversionrate of soil organic matter [16]. On these grounds, the level of enzyme activities couldcharacterize the demand for soil microbial nutrients and their relationship with the nutrientsupply level. Although the relationship between soil enzyme activities and soil organiccarbon components has been widely concerned, what are the effects of close-to-naturetransformation on soil organic carbon components and enzyme activities? Is it beneficialto improve soil carbon sequestration and stability of stand? And what is the relationshipbetween soil organic carbon fractions and enzyme activities and soil environmental factors?There is still no in-depth discussion and understanding of these issues.

Therefore, in order to elucidate the change characteristics of soil organic carboncomponents in P. massoniana and C. lanceolata plantations after CNT, the soil organic carboncomponents and enzyme activities of P. massoniana and C. lanceolata close-to-nature forests(PCN and CCN) and their control plantations (PCK and CCK) were compared in this study.We hypothesized that: (1) Compared with the control forests, CNT can promote soil organiccarbon and stability; (2) The change of soil organic carbon was regulated by enzyme activity.The purpose of this study is to provide a scientific basis and decision-making referencefor improving the understanding of the carbon cycle mechanism of plantations and thescientific management of the plantation ecosystem in the process of realizing dual carbongoals under the background of global change.

2. Materials and Methods2.1. Site Description

The study site was conducted at the Guangxi Youyiguan Forest Ecosystem ResearchStation, the Experimental Center of Tropical Forestry, Chinese Academy of Forestry (22◦10′ N,

Forests 2022, 13, 872 3 of 13

106◦50′ E), Pingxiang city, Guangxi Zhuang Autonomous Region in southern China. Theregion belongs to a subtropical monsoon climate, with an annual rainfall of 1200–1500 mmand an annual temperature of 20.5–21.7 ◦C. The soil is classified as an Oxisol in the USDAtaxonomy [17]. The subtropical evergreen broad-leaved forest is the main vegetation typein this region.

2.2. Experimental Design

The experiment was conducted in the plantations of P. massoniana and C. lanceolatawith an initial planting density of 2500 plants ha−1 in 1993. In 2004, the thinning wascarried out, and the retention density was 1200 plants ha−1. In 2007, half of the plantationswere transformed into mixed coniferous and broad-leaved uneven-aged plantations bythinning to 450 plants ha−1. In early 2008, two-year-old seedlings of Castanopsis fissa andErythrophleum fordii were interplanted in the transformed plantations of P. massoniana andC. lanceolata, and the replanting density of C. fissa and E. fordii was both 375 plants ha−1. Thenon-transformed plantations of P. massoniana and C. lanceolata, which have the same plantdensity as the transformed plantations considered as the controls. There were four plan-tation types, including non-transformed C. lanceolata pure plantation (CCK), transformedC. lanceolata plantation (CCN), non-transformed P. massoniana pure plantation (PCK), andtransformed P. massoniana plantation (PCN). Each plantation type was replicated four times,with an area of 20 m × 20 m for each plot. The stand characteristics are shown in Table 1.

Table 1. The stand characteristics in four plantations measure in August 2018.

Plantation Types CCK CCN PCK PCN

Slope aspect Southwest Southwest Northwest NorthwestSlope (◦) 24.6 ± 2.6 a 23.1 ± 2.9 a 21.3 ± 3.6 a 22.4 ± 4.1 a

Canopy density 0.78 ± 0.09 b 0.79 ± 0.11 b 0.71 ± 0.09 b 0.88 ± 0.03 aBasal area (m2 ha−1) 31.98 ± 2.18 b 50.82 ± 1.21 c 51.81 ± 0.21 a 61.43 ± 0.90 d

Litterfall (t ha−1 yr−1) 9.02 ± 0.19 b 9.54 ± 0.34 b 10.23 ± 0.94 a 10.84 ± 0.49 aCCK, CCN, PCK, and PCN represent the control and close-to-nature transformation C. lanceolata plantation andthe control and close-to-nature transformation of P. massoniana plantation, respectively. Data were shown asmeans ± se (n = 4). Values with different letters indicate significant plantation effects at p < 0.05.

2.3. Samplings and Measurements

Soil samples were collected in August 2018 to determine soil chemical properties. Intotal, 144 soil sampling points were randomly selected within the 16 sampling plots in eachof the 4 stands. The fresh and partially decomposed litter on the surface were removed,and then soil samples were collected at three depth, 0–10, 10–30, and 30–50 cm. A total of27 soil cores were collected using an 8.7 cm diameter stainless steel core from each plotand bulked to one composite sample per depth. Each soil sample was divided into twoparts: one part (0–50 cm) was naturally air-dried to determine the soil’s organic carboncomponents and chemical properties of the soil, and the other part (0–10 cm) was stored ina refrigerator at 4 ◦C for determining the soil enzyme activities, ammonium nitrogen, andnitrate nitrogen.

The content of soil organic carbon (SOC) was measured by K2Cr2O7 outside heatingmethod [18]; soil pH was measured in deionized water using a glass electrode. Totalphosphorus (TP) was measured by sodium hydroxide alkali fusion-molybdenum an-timony colorimetry, and available phosphorus (AP) was measured by the ammoniumfluoride-hydrochloric acid extraction method [19]. Total nitrogen (TN), ammonium ni-trogen (NH4

+-N), and nitrate nitrogen (NO3−-N) of soil were measured by the Kjeldahl

method and spectrophotometry, respectively, and alkali-hydrolyzable nitrogen (AN) wasmeasured by alkaline hydrolysis diffusion method [18]. Total potassium (TK) was measuredby sodium hydroxide alkali fusion method and flame photometer, and available potassium(AK) was measured by ammonium acetate extraction method and flame photometer [20].

Forests 2022, 13, 872 4 of 13

Particle organic carbon (POC) was measured with [(NaPO3)6] as the dispersant forseparation, which was dispersed by hand shaking (15 min) and oscillator (90 r min−1, 18 h)respectively, and dried by a 53 µm sieve (60 ◦C) and then analyzed with an elementalanalyzer [21]. Readily oxidation organic carbon (ROOC) was measured by the KMnO4oxidation method [22]. Water-soluble organic carbon (WSOC) was extracted by KCL andthen measured with a TOC instrument [17], and recalcitrant organic carbon (ROC) wasdigested with hydrochloric acid [23].

Catalase (CAT) activities were measured by ultraviolet spectrophotometry [24], pro-tease (PRO) activities were measured by casein colorimetry [25], urease (URE) activitieswere measured by sodium phenol colorimetry [26], and acid phosphatase (ACP) activitieswere measured by sodium diphenyl phosphate colorimetry [27].

2.4. Statistical Analyses

The analysis of environmental factors affecting SOC and its fractions was conductedby using the “vegan”, “agricolae”, and “plyr” packets. The analysis process is completed onR 4.1.0 [28]. The differences in chemical properties of different stands were tested by the useof one-way analysis of variance (one-way ANOVA) and least significant difference (LSD).The correlation between soil enzyme activities, organic carbon fractions, and physical andchemical properties was tested by Pearson correlation analysis. Redundancy analysis (RDA)with Canoco 4.5 software (Microcomputer Power, Inc., Ithaca, NY, USA) was carried outfor soil chemical properties, enzyme activities, soil organic carbon, and its fractions. All thestatistics in this paper conform to normal distribution and meet the variance homogeneitytest. The statistical significance test was set as p < 0.05.

3. Results3.1. Soil Chemical Properties

Soil organic carbon, total nitrogen, alkali-hydrolyzable nitrogen, total phosphorus, andavailable phosphorus of all stands are decreasing with the increase in soil layers, and thevertical distribution is obvious. The contents of soil organic carbon, total nitrogen, alkali-hydrolyzable nitrogen, total phosphorus, nitrate nitrogen, and ammonium nitrogen atdifferent soil levels of CNT were greater than those of the control forests in the C. lanceolataplantations. The contents of soil organic carbon, total phosphorus, nitrate nitrogen, andammonium nitrogen of transformed P. massoniana plantations were higher than those ofthe control plantations. However, no significant difference was detected in the contentsof all nutrients of P. massoniana and C. lanceolata between the close to natural and controlforests (Table 2).

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Table 2. Effects of close-to-nature transformation on soil chemical properties of Pinus—Cuninghamiaplantations. Mean ± SE.

SoilChemical Properties Soil Depth (cm) CCK CCN PCK PCN

pH0–10 4.77 ± 0.14 Aa 4.80 ± 0.04 Aa 4.24 ± 0.06 Ba 4.25 ± 0.04 Ba

10–30 4.67 ± 0.08 Aa 4.64 ± 0.07 Aa 4.31 ± 0.12 Aa 4.29 ± 0.11 Aa30–50 4.74 ± 0.10 Aa 475 ± 0.08 Aa 4.33 ± 0.05 ABa 4.48 ± 0.08 Ba

Organic carbon (g kg−1)0–10 18.45 ± 2.43 Ba 20.40 ± 1.20 ABa 26.65 ± 1.70 Aa 27.99 ± 2.18 Aa

10–30 9.52 ± 1.77 Bb 12.41 ± 1.72 ABb 16.58 ± 1.19 Ab 16.82 ± 1.10 Ab30–50 8.83 ± 1.73 Ab 8.87 ± 0.15 Ab 8.20 ± 0.93 Ac 9.28 ± 1.71 Ac

Total nitrogen (g kg−1)0–10 1.27 ± 0.04 Ba 1.28 ± 0.05 Ba 1.56 ± 0.06 Aa 1.58 ± 0.09 Aa

10–30 0.59 ± 0.14 Ab 0.77 ± 0.05 Ab 0.93 ± 0.10 Ab 0.92 ± 0.04 Ab30–50 0.53 ± 0.06 Ab 0.69 ± 0.11 Ab 0.66 ± 0.07 Ab 0.61 ± 0.17 Ab

Alkali-hydrolyzablenitrogen (mg kg−1)

0–10 81.06 ± 6.45 Aa 92.21 ± 9.81 Aa 99.87 ± 2.40 Aa 97.82 ± 4.8 Aa10–30 53.02 ± 7.83 Ab 60.55 ± 3.99 Ab 59.98 ± 5.35 Ab 57.50 ± 3.93 Ab30–50 32.26 ± 6.87 Ab 46.71 ± 2.59 Ab 50.68 ± 3.50 Ab 53.73 ± 5.72 Ab

Total phosphorus(g kg−1)

0–10 0.23 ± 0.06 Aa 0.26 ± 0.02 Aa 0.26 ± 0.01 Aa 0.27 ± 0.01 Aa10–30 0.20 ± 0.04 Aa 0.21 ± 0.03 Aa 0.20 ± 0.01 Ab 0.20 ± 0.02 Aab30–50 0.18 ± 0.04 Aa 0.20 ± 0.04 Aa 0.19 ± 0.02 Ab 0.19 ± 0.03 Ab

Available phosphorus(mg kg−1)

0–10 1.15 ± 0.12 Ba 1.08 ± 0.20 ABa 1.81 ± 0.22 Aa 1.70 ± 0.14 ABa10–30 0.50 ± 0.09 Bb 0.80 ± 0.15 ABa 1.03 ± 0.12 Ab 1.05 ± 0.10 Ab30–50 0.35 ± 0.08 Ab 0.51 ± 0.04 Aa 0.40 ± 0.08 Ac 0.49 ± 0.13 Ac

Total potassium(g kg−1)

0–10 20.01 ± 2.64 Aa 16.02 ± 3.81 AB 6.45 ± 1.53 Ba 5.81 ± 0.50 Ba10–30 19.87 ± 2.98 Aa 14.70 ± 1.93 Aa 7.04 ± 0.70 Ba 6.22 ± 0.31 Ba30–50 20.71 ± 6.62 Aa 14.29 ± 2.48 ABa 5.80 ± 0.27 Ba 5.56 ± 0.58 Ba

Available potassium(mg kg−1)

0–10 125.59 ± 11.80 Aa 106.38 ± 20.21 ABa 72.98 ± 19.31 ABa 51.33 ± 10.70 Ba10–30 73.68 ± 24.47 Aa 96.12 ± 17.78 Aa 38.78 ± 18.55 Aa 24.39 ± 6.11 Aa30–50 120.72 ± 24.55 Aa 103.65 ± 25.35 ABa 34.80 ± 18.55 Ba 22.37 ± 5.75 Ba

NO3−-N (mg kg−1) 0–10 18.58 ± 0.53 A 20.74 ± 1.63 A 18.27 ± 1.14 A 19.61 ± 0.97 A

NH4+ -N (mg kg−1) 0–10 20.57 ± 1.09 A 24.05 ± 3.01 A 22.08 ± 1.99 A 23.28 ± 2.63 A

CCK, CCN, PCK, and PCN represent the control and close-to-nature transformation of C. lanceolata plantation andthe control and close-to-nature transformation of P. massoniana plantation, respectively. Different small lettersmeant significant differences among different soil layers in the same stands, and different capital letters meantsignificant differences among different stands in the same soil layers at p < 0.05.

3.2. Soil Organic Carbon Fractions

The four kinds of organic carbon content in the same stand had obvious verticaldistribution characteristics. Except for readily oxidation organic carbon, they all decreasesignificantly with the increase in soil depth. Four organic carbons in transformed planta-tions are higher than those in the control plantations (except for 10–30 cm, readily oxidationorganic carbon) in C. lanceolata plantations. In the 0–10 cm soil layer, the content of water-soluble organic carbon in the transformed plantations is 81.29%, significantly higher thanthose in the control stands, and in the 10–30 cm soil layer, the contents of particulateorganic carbon and water-soluble organic carbon in the transformed plantations are 95.42%and 48.68% significantly higher than those in the control stands, respectively (p < 0.05).However, there is no obvious change law for the contents of four organic carbons inP. massoniana plantations, nor a significant difference between the reference plantations andthe transformed plantations. In general, regardless of whether it is transformed or not,the four soil organic carbon fractions of P. massoniana plantations are higher than thoseof C. lanceolata plantations, and some are significantly higher than those of C. lanceolataplantations (Figure 1).

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Figure 1. Characteristics of soil organic carbon fractions after close-to-nature transformation. CCK,

CCN, PCK, and PCN represent the control and close-to-nature transformation of C. lanceolata plan-

tation and the control and close-to-nature transformation of P. massoniana plantation, respectively.

Different small letters meant significant differences among different soil layers in the same stands,

and different capital letters meant significant differences among different stands in the same soil

layers at p < 0.05.

3.3. Soil Enzyme Activities

The activities of protease, urease, and acid phosphatase in transformation C. lanceo-

lata plantation are 5.62%, 18.75%, and 28.40% higher than in control stands, and in P. mas-

soniana plantations after transformation, the activities of protease and catalase are 7.37%

and 6.00% higher than control stands, respectively, while the activities of urease and acid

phosphatase are reduced (Figure 2). Except for protease, the changes in the activities of

other enzymes in C. lanceolata and P. massoniana plantations are exactly opposite, but the

activities of four enzymes in the 0–10 cm soil layer of C. lanceolata and P. massoniana plan-

tations (p > 0.05) are not significantly affected by the CNT.

Figure 1. Characteristics of soil organic carbon fractions after close-to-nature transformation. CCK,CCN, PCK, and PCN represent the control and close-to-nature transformation of C. lanceolata plan-tation and the control and close-to-nature transformation of P. massoniana plantation, respectively.Different small letters meant significant differences among different soil layers in the same stands,and different capital letters meant significant differences among different stands in the same soillayers at p < 0.05.

3.3. Soil Enzyme Activities

The activities of protease, urease, and acid phosphatase in transformation C. lanceolataplantation are 5.62%, 18.75%, and 28.40% higher than in control stands, and in P. massonianaplantations after transformation, the activities of protease and catalase are 7.37% and 6.00%higher than control stands, respectively, while the activities of urease and acid phosphataseare reduced (Figure 2). Except for protease, the changes in the activities of other enzymesin C. lanceolata and P. massoniana plantations are exactly opposite, but the activities of fourenzymes in the 0–10 cm soil layer of C. lanceolata and P. massoniana plantations (p > 0.05) arenot significantly affected by the CNT.

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Figure 2. Characteristics of soil enzyme activities after close-to-nature transformation (0–10 cm).

CCK, CCN, PCK, and PCN represent the control and close-to-nature transformation of C. lanceolata

plantation and the control and close-to-nature transformation of P. massoniana plantation, respec-

tively.

3.4. Correlation Analysis of Organic Carbon Fractions with Enzyme Activities and Chemical

Factors

There was no correlation between catalase and water-soluble organic carbon, but a

positive correlation between catalase and other organic carbon components and a signifi-

cant positive correlation between protease and recalcitrant organic carbon. Soil pH, total

potassium, and available potassium are significantly negatively correlated with organic

carbon fractions. Total nitrogen is significantly positively correlated with organic carbon

fractions, alkali-hydrolyzable nitrogen is significantly positively correlated with recalci-

trant organic carbon, and available phosphorus is significantly positively correlated with

water-soluble organic carbon, readily oxidation organic carbon, and particulate organic

carbon. Acid phosphatase, total phosphorus, ammonium nitrogen, and nitrate nitrogen

are not significantly correlated with organic carbon fractions (Table 3).

RDA was used to determine the relationship among the organic carbon fractions, soil

chemical properties, and enzyme activities (Figure 3). The result showed that the varia-

tions in the cumulative interpretation of the first and second axes were 76.3%. The first

axis indicates the variables in 64.9%, and the second axis explains the variables in 11.4%.

Permutation tests showed that the variations observed in overall organic carbon fractions

were best explained by the vectors of soil chemical properties (pH, total nitrogen, total

potassium, available potassium, and available phosphorus) (p < 0.05). The change in the

content of organic carbon fractions was negatively correlated with pH, total potassium,

and available potassium and was positively correlated with the total nitrogen and availa-

ble phosphorus.

Figure 2. Characteristics of soil enzyme activities after close-to-nature transformation (0–10 cm).CCK, CCN, PCK, and PCN represent the control and close-to-nature transformation of C. lanceolataplantation and the control and close-to-nature transformation of P. massoniana plantation, respectively.

3.4. Correlation Analysis of Organic Carbon Fractions with Enzyme Activities andChemical Factors

There was no correlation between catalase and water-soluble organic carbon, but apositive correlation between catalase and other organic carbon components and a signifi-cant positive correlation between protease and recalcitrant organic carbon. Soil pH, totalpotassium, and available potassium are significantly negatively correlated with organiccarbon fractions. Total nitrogen is significantly positively correlated with organic carbonfractions, alkali-hydrolyzable nitrogen is significantly positively correlated with recalci-trant organic carbon, and available phosphorus is significantly positively correlated withwater-soluble organic carbon, readily oxidation organic carbon, and particulate organiccarbon. Acid phosphatase, total phosphorus, ammonium nitrogen, and nitrate nitrogen arenot significantly correlated with organic carbon fractions (Table 3).

Table 3. Correlation coefficients of soil enzyme activity, organic carbon fractions, and chemicalfactors (0–10 cm).

CAT PRO URE ACP pH TN AN TP AP TK AK NH4+-N NO3−-N

SOC 0.61 * 0.44 0.49 −0.35 −0.87 *** 0.76 *** 0.45 −0.21 0.50 −0.68 ** −0.67 ** 0.08 0.01WSOC 0.42 0.25 0.44 −0.22 −0.55 * 0.50 * 0.50 −0.07 0.52 * −0.59 * −0.54 * 0.03 0.17ROOC 0.59 * 0.13 0.39 −0.49 −0.76 *** 0.65 ** 0.41 −0.03 0.66 ** −0.59 * −0.58 * 0.14 0.05ROC 0.56 * 0.53 * 0.41 −0.26 −0.83 *** 0.73 ** 0.54 * −0.26 0.37 −0.65 ** −0.63 ** 0.05 −0.01POC 0.61 * 0.40 0.47 −0.45 −0.83 *** 0.69 ** 0.40 −0.00 0.51 * −0.66 ** −0.63 ** −0.04 −0.20

* p < 0.05; ** p < 0.01; *** p < 0.001. SOC: Total organic carbon; WSOC: Water-soluble organic carbon; ROOC:Readily oxidation organic carbon; ROC: Recalcitrant organic carbon; POC: Particulate organic carbon; CAT:Catalase; PRO: Protease; URE: Urease; ACP: Acid phosphatase; TN: Total nitrogen; TN: Total nitrogen; AN: Alkali-hydrolyzable nitrogen; TP: Total phosphorus; AP: Available phosphorus; TK: Total potassium; AK: Availablepotassium; NH4

+-N: Ammonium nitrogen; NO3−-N: Nitrate nitrogen.

Forests 2022, 13, 872 8 of 13

RDA was used to determine the relationship among the organic carbon fractions, soilchemical properties, and enzyme activities (Figure 3). The result showed that the variationsin the cumulative interpretation of the first and second axes were 76.3%. The first axisindicates the variables in 64.9%, and the second axis explains the variables in 11.4%. Per-mutation tests showed that the variations observed in overall organic carbon fractions werebest explained by the vectors of soil chemical properties (pH, total nitrogen, total potassium,available potassium, and available phosphorus) (p < 0.05). The change in the content oforganic carbon fractions was negatively correlated with pH, total potassium, and availablepotassium and was positively correlated with the total nitrogen and available phosphorus.

Forests 2022, 13, x FOR PEER REVIEW 8 of 13

Figure 3. Redundancy analysis (RDA) of soil chemical properties and enzyme activities on soil or-

ganic carbon and its organic carbon fractions. Organic carbon fractions (SOC: Total organic carbon;

WSOC: Water-soluble organic carbon; ROOC: Readily oxidation organic carbon; ROC: Recalcitrant

organic carbon; POC: Particulate organic carbon) are the response variables, and soil chemical prop-

erties and enzyme activities (PRO: Protease; URE: Urease; ACP: Acid phosphatase; TN: Total nitro-

gen; TN: Total nitrogen; AN: Alkali-hydrolyzable nitrogen; AP: Available phosphorus; TK: Total

potassium; AK: Available potassium) are explanatory variables. CCK, CCN, PCK, and PCN repre-

sent the control and close-to-nature transformation of C. lanceolata plantation and the control and

close-to-nature transformation of P. massoniana plantation, respectively.

Table 3. Correlation coefficients of soil enzyme activity, organic carbon fractions, and chemical fac-

tors (0–10 cm).

CAT PRO URE ACP pH TN AN TP AP TK AK NH4+-N NO3−-N

SOC 0.61 * 0.44 0.49 −0.35 −0.87 *** 0.76 *** 0.45 −0.21 0.50 −0.68 ** −0.67 ** 0.08 0.01

WSOC 0.42 0.25 0.44 −0.22 −0.55 * 0.50 * 0.50 −0.07 0.52 * −0.59 * −0.54 * 0.03 0.17

ROOC 0.59 * 0.13 0.39 −0.49 −0.76 *** 0.65 ** 0.41 −0.03 0.66 ** −0.59 * −0.58 * 0.14 0.05

ROC 0.56 * 0.53 * 0.41 −0.26 −0.83 *** 0.73 ** 0.54 * −0.26 0.37 −0.65 ** −0.63 ** 0.05 −0.01

POC 0.61 * 0.40 0.47 −0.45 −0.83*** 0.69** 0.40 −0.00 0.51 * −0.66 ** −0.63 ** −0.04 −0.20

* p < 0.05; ** p < 0.01; *** p < 0.001. SOC: Total organic carbon; WSOC: Water-soluble organic

carbon; ROOC: Readily oxidation organic carbon; ROC: Recalcitrant organic carbon; POC: Par-

ticulate organic carbon; CAT: Catalase; PRO: Protease; URE: Urease; ACP: Acid phosphatase;

TN: Total nitrogen; TN: Total nitrogen; AN: Alkali-hydrolyzable nitrogen; TP: Total phospho-

rus; AP: Available phosphorus; TK: Total potassium; AK: Available potassium; NH4+-N: Am-

monium nitrogen; NO3−-N: Nitrate nitrogen.

4. Discussion

In this study, the content of soil organic carbon in C. lanceolata and P. massoniana plan-

tations after close-to-nature transformation was higher than that of their control planta-

tions in 2018 (10 years after the CNT). At the same time, after the transformation, the dif-

ference in available phosphorus content in the C. lanceolata plantations was reduced, while

the available phosphorus content in deep soil was increased (Table 2), and other nutrient

Figure 3. Redundancy analysis (RDA) of soil chemical properties and enzyme activities on soil organiccarbon and its organic carbon fractions. Organic carbon fractions (SOC: Total organic carbon; WSOC:Water-soluble organic carbon; ROOC: Readily oxidation organic carbon; ROC: Recalcitrant organiccarbon; POC: Particulate organic carbon) are the response variables, and soil chemical propertiesand enzyme activities (PRO: Protease; URE: Urease; ACP: Acid phosphatase; TN: Total nitrogen; TN:Total nitrogen; AN: Alkali-hydrolyzable nitrogen; AP: Available phosphorus; TK: Total potassium;AK: Available potassium) are explanatory variables. CCK, CCN, PCK, and PCN represent the controland close-to-nature transformation of C. lanceolata plantation and the control and close-to-naturetransformation of P. massoniana plantation, respectively.

4. Discussion

In this study, the content of soil organic carbon in C. lanceolata and P. massoniana plan-tations after close-to-nature transformation was higher than that of their control plantationsin 2018 (10 years after the CNT). At the same time, after the transformation, the differencein available phosphorus content in the C. lanceolata plantations was reduced, while theavailable phosphorus content in deep soil was increased (Table 2), and other nutrient con-tents increased to varying degrees. Previous studies in 2010 (2 years after the CNT) showedthat the content of available phosphorus in the transformed C. lanceolata plantations wassignificantly lower than that in the reference plantations, and the content of soil organiccarbon in the transformed P. massoniana plantations was significantly lower than that inthe reference plantations [12]. It can be found that the loss of soil organic carbon and some

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nutrient elements caused by the early close-to-nature transformation has been improved,which is conducive to the sequestration of soil carbon and the accumulation of nutrients inthe plantations. Different study durations will also lead to different study results. The CNTtime is not long enough so that the difference among stands with different treatments isnot significantly different, and the transformation effect is not obvious. However, with thepassage of time, the productivity of stand after CNT will gradually increase, which willgradually increase the input of soil organic matter and reduce the decomposition rate oforganic matter [29]. Therefore, the long-term effects of close-to-nature transformation onthe content of organic carbon at different stages need to be continuously studied.

The readily oxidation organic carbon content of soil in C. lanceolata and P. massonianaplantations decreased first and then increased with the soil depth. This may be because thereadily oxidation organic carbon is more easily oxidized and decomposed in the soil, and thedecomposition products move from the topsoil layer to the lower layer with water, whichare sequestrated by the microorganisms in the lower layer and then become a potentialcarbon source for its growth [30], thereby increasing the accumulation of readily oxidationorganic carbon in the deep layer. In contrast, no significant difference was observed inreadily oxidation organic carbon tween the transformed C. lanceolata and P. massonianaplantations and their control plantations. On the one hand, it may be the result of mutualtransformation between readily oxidation organic carbon and other forms of organic carbonin the sample plot, which maintains the stable fluctuation of the readily oxidation organiccarbon content in soil [31]. On the other hand, the readily oxidized organic carbon was notsensitive to the change in tree species composition. However, the more detailed reasonsneed to be further analyzed.

Particulate organic carbon in soil can quickly and effectively reflect the changes in over-all soil quality so that it can be used as a sensitive indicator of changes in soil organic carbonpool and can also indicate the evolution of soil fertility [32]. The content of particulateorganic carbon in the close-to-nature transformation plantation at 10–30 cm soil layer wassignificantly higher than that in the control plantation. The results showed that the CNTwas not only beneficial to the accumulation of soil organic carbon but also could effectivelyimprove soil quality. However, there was no significant difference in particulate organiccarbon content between topsoil and deep soil. The reason may be that the topsoil layer hasa large amount of litterfall and incomplete decomposition, and the failure to remove thesemi-decomposed litterfall during sample collection and subsequent treatment, resultingin little difference in the particulate organic carbon content of the topsoil in the sampleplot [31]. Moreover, the deep soil has a weaker agglomeration capacity and fewer rootexudates [31]. Therefore, the content of particulate organic carbon between the transformedplantations and the reference plantations is not significantly different.

Recalcitrant organic carbon is an organic carbon fraction that has existed in the soilfor a long time and is difficult to be decomposed and utilized by microorganisms, and itis also a reserve carbon source for microbial decomposition [33]. In this study, it is foundthat recalcitrant organic carbon and organic carbon have the highest correlation coefficient(0.97), which is consistent with previous study results [34]. Recalcitrant organic carbonis closely related to the accumulation of organic carbon. Alkali-hydrolyzable nitrogen,available phosphorus, total potassium, and available potassium showed consistent changeswith recalcitrant organic carbon in P. massoniana plantations after CNT, suggesting that soilavailable nutrients may be correlated with soil organic carbon components. The chemicalproperties of soil (pH, total nitrogen, total potassium, available potassium) in Table 3 aresignificantly correlated with soil organic carbon and organic carbon fractions. Therefore,whether the change of soil chemical properties is an important factor causing the changeof soil carbon components in stands with different treatments is worth further study. Atthe same time, the decrease in recalcitrant organic carbon also indicates that the CNThas an impact on the stability of soil carbon, which intends to promote the conversion ofrecalcitrant organic carbon to active fractions [35]. The correlation analysis in this study also

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shows that recalcitrant organic carbon is significantly positively correlated with particulateorganic carbon and readily oxidation organic carbon (Table 3).

The results revealed that soil pH and total nitrogen were the main environmentalfactors that drive the variation of organic carbon and its fractions (Figure 3). This isconsistent with the results of many previous studies that soil pH can change the organiccarbon by affecting microorganisms [36]. The study by Liao Dan et al. [37] also showedthat the contents of activated carbon, chronic carbon, and recalcitrant organic carbon werehighly significantly positively correlated with TN. With the higher N content, the soil carbonpooling can be significantly increased, the carbon:nitrogen ratio can be reduced, and themicrobial activity can be improved. With the increase in soil nitrogen, the decompositionof organic carbon can be accelerated as well [38]. In this study, the total nitrogen contentin the soil of transformed C. lanceolata plantations was higher than that of the referenceplantations, while the P. massoniana forest plantations showed differently, which may alsobe one of the reasons for the different response of C. lanceolata and P. massoniana plantationsto organic carbon and its fractions. Generally, after the CNT, the nutrient content of forestsoil will be gradually accumulated, and the soil organic matter with high nitrogen contentwas easily decomposed, transported, and transformed by microorganisms within a certainrange [39], and then affects the process of soil organic carbon sequestration.

Organic carbon is taken as the carrier of enzymes. With the high-level organic carboncontent, the production of extracellular enzymes can be induced, and the synthesis ofenzymes and the growth and reproduction of microorganisms can be promoted [40]. Theresults of this study showed that the activities of protease, urease, and acid phosphatasein C. lanceolata plantations after transformation are 5.62%, 18.75%, and 28.40% higher thancontrol stands, and the activities of protease and catalase activities in P. massoniana planta-tions are 7.37% and 6.00% higher than control plantations. Through the CNT, the contentof readily decomposable organic carbon in the stands was increased, thus promoting theutilization of readily decomposable carbon by microorganisms. This can be explained bythe fact that the contents of water-soluble organic carbon and particulate organic carbon inFigure 2 were significantly higher than that of the reference plantations. It is found in thestudy that the impact of soil organic carbon on enzyme activity was closely related to thetype of organic carbon [41]. This study showed that catalase was significantly correlatedwith organic carbon fractions (except water-soluble organic carbon), and protease wassignificantly correlated with recalcitrant organic carbon, indicating that organic carbon wassensitive to changes in the activities of these two enzymes, and the strength of enzymeactivities also affected its mineralization rate. Different from the above two enzymes,acid phosphatase and urease were not significantly correlated with organic carbon and itsfractions, indicating that their impact on soil organic carbon fractions may depend moreon microbial species, biomass, and activities. Moreover, the sampling was conducted inthe rainy season, and the soil moisture of the four stands was similar so that acid phos-phatase was more vulnerable to soil moisture than organic carbon [42]. However, due tothe coupling relationship between various nutrient elements in the soil, it was uncertainwhether the higher enzyme activities led to the increase in soil organic carbon contentor whether the larger soil organic carbon pool provided sufficient substrate for enzymeactivities, thus promoting the improvement of enzyme activity [43]. Most scholars believedthat soil enzyme activities were simultaneously regulated by key physical and chemicalproperties of soil, soil microbial biomass, and soil organic matter content [44]. It is foundin the study of Sinsabaugh et al. [45] that higher nitrogen availability would promote thedegradation of unstable organic carbon. In addition, seasonal changes and microflorachanges affected soil carbon pool storage and enzyme activities. Due to sampling only inthe growing season, this study lacks relevant data to study seasonal changes and microbialcommunity structure. This may also be an important reason why there is no significantdifference in enzyme activities between the transformed plantations and the referenceplantations in this study. Therefore, in order to better explore the relationship between soil

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organic carbon fractions and enzyme activities, the seasonal dynamics and microorganismsneed to be further studied in the future.

5. Conclusions

Close-to-nature transformation can improve the soil organic carbon and enzymeactivity in coniferous plantation, and the change components mainly are active carbon,which is closely related to soil enzyme activities and soil chemical properties, amongthem, soil pH, total nitrogen and catalase were the main factors affecting organic carboncomponents. The results showed that close-to-nature transformation will promote soilorganic carbon accumulation by changing the composition of tree species but had a negativeeffect on soil organic carbon stability to a certain extent. Therefore, the effect of tree speciesconfiguration on soil carbon stability components should be considered in the practice offorest management.

Author Contributions: Conceptualization, A.M. and J.Z. (Ji Zeng); methodology, A.M.; software,W.S.; validation, W.S. and A.M.; formal analysis, A.M. and J.Z. (Ji Zeng); investigation, H.L., H.M.,J.M., K.Y., Z.L. (Zhongguo Li) and J.Z. (Jihui Zhang); resources, Z.L. (Zhaoying Li); data curation, W.S.and Y.T.; writing—original draft preparation, W.S.; writing—review and editing, W.S.; visualization,W.S.; supervision, J.W.; project administration, J.Z. (Ji Zeng); funding acquisition, A.M. and J.Z.(Ji Zeng). All authors have read and agreed to the published version of the manuscript.

Funding: This work was supported by the National Natural Science Foundation of China (32071764),National Natural Science Foundation of Guangxi Province (2020GXNSFAA297208), FundamentalResearch Funds of the Chinese Academy of Forestry (CAFYBB2021ZW001-2) and the NationalNatural Science Foundation of China (31971655).

Acknowledgments: We gratefully acknowledge the guidance of Junkun Lu from Research Institute ofTropical Forestry, China Academy of Forestry, Hui Wang from Institute of Forest Ecology, Environmentand Nature Conservation, Chinese Academy of Forestry, and You Yeming, Xiaoguo Zhou fromGuangxi University in manuscript writing and revision; as well as Hai Chen and Maofeng Zhu forassistance in field work and laboratory analysis.

Conflicts of Interest: The authors declare no conflict of interest.

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