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Europ. J. Agronomy 27 (2007) 62–71

Labile phosphorus forms in irrigated and rainfed semiarid Mediterraneangrassy crops with long-term organic or conventional farming practices

Joan Romanya a,∗, Pere Rovira b

a Department of Natural Products Plant Biology and Soil Science, Universitat de Barcelona, Avgda. Joan XXIII s/n, 08028 Barcelona, Spainb Department of Plant Biology, Universitat de Barcelona, Avgda. Diagonal 645, 08028 Barcelona, Spain

Received 25 October 2006; received in revised form 2 February 2007; accepted 9 February 2007

bstract

The availability of organic fertilisers plays a major role in organic farming systems. Such systems exclude the use of synthetic fertilisers, whilstiming to optimise internal nutrient cycling. The low availability of manures, particularly in dry areas, can lead to negative nutrient balances in manyrganic farming systems. Such negative nutrient balances are mainly found for P and K. In this paper, we aim to study the availability of P in irrigatednd rainfed semiarid Mediterranean grassy crops with long-term organic and conventional farming practices. NaHCO3 extracts were prepared fromn array of soils from 16 plots, covering organic and conventional management in rainfed and irrigated conditions. Inorganic (NaHCO3-Pi) andrganic P (NaHCO3-Po) were analysed in the extracts and related to soil properties (carbonate content, pH, organic C and N content). Rainfed,rganically managed soils showed low P availability compared to conventionally managed soils. However, organically managed irrigated soilshowed slightly higher NaHCO3-Pi than conventional soils. This is due to the fertilisation regime applied to the irrigated, organically managedystems. Such systems received four-fold the amount of manure applied to organically managed rainfed soils. In soils with low P availability,aHCO3-Pi was largely depleted while NaHCO3-Po remained nearly unchanged. In soils with good or moderate P availability, NaHCO3-Pi

ppeared mainly to be regulated by soil organic matter (organic C and N). In conditions of low P availability, NaHCO3-Pi was mainly regulated by

hemical processes related to soil pH and carbonate content. The regulation of NaHCO3-Po was less clear. Under low P availability, NaHCO3-Piepletion occurred mainly in soils with high organic C and N and low pH. In low P soils with a high pH and carbonate content, NaHCO3-Pippeared to be geochemically protected. In calcareous soils, management practices need to increase or maintain the level of soil organic C toacilitate mobilisation of the P reserve.

2007 Elsevier B.V. All rights reserved.

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eywords: P availability; Organic P; NaHCO3-P; Soil fertility

. Introduction

Organic farming aims to provide a sustainable alternative tontensive agricultural systems (Stockdale et al., 2001). The man-gement of organic farms excludes the use of synthetic fertilisersnd pesticides, while increasing and maintaining soil fertilityver the long-term (IFOAM, 2000). To achieve this, organicarming enhances internal nutrient cycling by: incorporatingrop residues, introducing crop rotations, using green manuresnitrogen fixing leys) and different types of organic fertilisers.

he scarcity of organic fertilisers (manures and slurries) in manygricultural areas often results in highly negative nutrient bal-nces in organically managed agricultural lands (Alfoeldi et al.,

∗ Corresponding author. Tel.: +34 93402 4494; fax: +34 93402 4495.E-mail address: jromanya@ub.edu (J. Romanya).

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161-0301/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.eja.2007.02.001

002). Negative balances may be particularly high in extensiverops in dry and semiarid regions with low cattle densities. Inuch areas there is low availability of farmyard manure. Nega-ive balances may occur for all major nutrients. However, theyre often specially large for P and K. Indeed, several authorsave reported decreases in soil P availability after years of con-inued organic management (Schjønning et al., 2002; Goslingnd Shepherd, 2005), or depletion of P from past fertilisation byrganically grown crops (Wivstad et al., 2005).

The effects of past P fertilisation on soil P testing and onrop productivity may last for several decades (Ellmer et al.,000; Dodd and Mallarino, 2005). Ellmer et al. (2000) foundhat 60 years without P fertilisation had little influence on the

vailability of P for plants, but decreased total soil P content.rops often use less than one third of applied inorganic fer-

iliser while large amounts of residual P accumulate in the soilAulakh et al., 2003). Underfertilised crops that grow in fields

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hat had previously received P fertiliser over the years may usehe accumulated soil P reserves. Legume crops can enhance soil

inputs by biological N fixation. However, P inputs to the sys-em are limited to fertilisation practices. Nevertheless, increasesn soil organic matter may enhance P desorption and mobilityy: enhancing soil microbial activity (Lee et al., 1990); increas-ng mycorrhizal symbiosis; reducing soil P sorption capacityIramuremye and Dick, 1996). Plants can also contribute toncreasing soil P solubility by excreting organic acids into thehizosphere and/or by lowering the soil pH in the case of alka-ine soils (Gerke and Meyer, 1995). In addition, P availability

ay have a major effect on soil microbial activity in P limitedoils (Tate and Salcedo, 1988). Thus, P may become crucial tooil nutrient cycling.

Arable alkaline Mediterranean soils are typically low inrganic C (often less than 1%). Organic management practicesn this area do not always result in raising these low levelsf soil C. Such soils contain large amounts of Ca in solution,n equilibrium with carbonates that often accumulate as sec-ndary carbonates (Yaalon, 1997). Under these conditions, theormation of secondary CaCO3 may precipitate P with Ca andhus affect the retention of P to a greater extent than Fe andl (Saavedra and Delgado, 2005; Carreira et al., 2006) and/oricrobial biomass (Lajtha and Schlesinger, 1988). In alkaline

oils, the precipitation of Ca phosphates increases with pH.his reduces the availability of P for plants and microbes, as

ndicated by decreases in microbial biomass and increasing pHMarschner et al., 2005).

In semiarid areas crop productivity and fertilisation regimere generally higher in irrigated than in rainfed soils. In con-equence, in organically managed crops inputs of exogenousrganic matter often are higher in irrigated than in rainfed soils.he aims of this study were to evaluate soil P availability in rain-

ed and irrigated semiarid Mediterranean organically managedrassy crops, in comparison to the soils of respective rainfednd irrigated conventionally managed crops that had regularlyeceived inorganic P fertilisers. We studied the effects of changesn soil properties (organic matter, pH and carbonates), which

ay have resulted from the different fertilisation regimes andarming practices, on labile inorganic and organic P mobilitynd on the contribution of labile organic P to P availability inhese systems.

. Methods

.1. Study area and management practices

The study was carried out in a set of agricultural fieldsn the Ebro river depression (north eastern Iberian Peninsula;1◦49′, −0◦2′E, 300 m.a.s.l.) where arable grassy crops haveeen grown for at least the last 40 years. During this time, somef the areas have been irrigated by surface flooding while theest of the agricultural land has remained rainfed. The climate

f the area is Mediterranean semiarid, with a mean annual airemperature of 14.4 ◦C and mean annual rainfall of 436.6 mm.oils are alkaline (pH values range from 8.1 to 9.2) clay loamnd sandy clay loam textures.

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Agronomy 27 (2007) 62–71 63

In the last 18 years, organic farming practices have been intro-uced in some rainfed and irrigated fields scattered in the areaf study. Both organic and conventional rainfed soils carriedbarley–fallow rotation. Conventional irrigated soils carriedwheat monoculture, while the organically managed irrigated

oils carried a wheat (oats)–peas rotation. At the time of sam-ling in 2004, all the irrigated sites were seeded with wheat andll the rainfed sites with barley.

Organic farming avoids the use of pesticides and syntheticertilisers. Thus, organically managed soils received an appli-ation of 5 Mg ha−1 every other year of composted poultryanure for the rainfed soils, and 10 Mg ha−1 yr−1 of the same

ype of manure for the irrigated soils. P fertilisation of theonventional fields consisted of 30–40 kg P ha−1 yr−1 of super-hosphate or a similar fertiliser in rainfed soils and between 50nd 60 kg P ha−1 yr−1 in irrigated soils. We cannot be accuratealculating the amount of P that was added in the organicallyanaged soils, because the amount of poultry manure that was

dded for the last two decades has not been precisely measurednd because we do not have the analyses of the specific poultryanure used. However based on general information on poultryanure with 0.6–0.8% of P and using the approximated figures

f the amounts applied we can tentatively estimate that the addi-ions of total P in these plots may be about 15–20 kg P ha−1 yr−1

or the rainfed and 60–80 kg P ha−1 yr−1 for the irrigated soils.

.2. Experimental design and soil sampling

Within the aforementioned agricultural area, eight indepen-ent agricultural fields were selected that had been subject torganic management schemes for the last 18 years. Four of theseelds were regularly irrigated, while the other four were rain-ed. Another set of eight fields (four irrigated and four rainfed)ith conventional management were selected near to the organicelds. In each field, a subplot of 625 m2 was selected and threeoil samples of 0–30 cm were taken from three points withinhe subplot. To describe the distribution of labile P in depthach soil sample was divided into three layers: 0–10, 10–20 and0–30 cm.

.3. Soil analyses

Soils were air-dried and sieved (2 mm) prior to soil analyses.subsample of each soil was ground for analyses of total organic, total N and carbonate content. Total organic C was deter-ined by dichromate oxidation; total N by the CHNS elemental

nalyser Carlo Erba NA-1500; total carbonates by measuringhe air pressure changes after adding an excess of hydrochloriccid to soil samples (Bernard’s calcimeter method). The air-ried mineral soil was analysed for pH in water (soil wateratio 1:2.5) and labile inorganic and organic P. Labile P wasetermined by the Olsen extraction with 0.5 M NaHCO3 (pH.5, soil solution ratio 1:20; see Kuo, 1996). The labile inor-

anic P (Pi) or molybdate reactive P was determined using theethod of Murphy and Riley (1962), after the extracts had been

eutralised with a dilute HCl solution. The labile organic PPo) or molibdate non reactive P in the NaHCO3 extract was

6 p. J. Agronomy 27 (2007) 62–71

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4 J. Romanya, P. Rovira / Euro

etermined by subtracting Pi from the total amount of P in thextract, which was determined by the Murphy and Riley (1962)ethod after digesting the extracts with K2S2O8 and NaOH in

he autoclave at 120 ◦C (Ebina et al., 1983). Although, this noneactive fraction may also include non organic P forms such asolyphospates we will refer to this fraction throughout the papers labile organic for the sake of simplicity and in concordanceith other studies on soil P forms (see for example Cross andchlesinger, 1995). For texture analyses within each subplot,oil layers were bulked by mixing soils from the three samplingoints to give one sample per layer and subplot. Texture wasnalysed by sieving and sedimentation by the Robinson pipetteethod.

.4. Statistical analyses

A complete factorial analysis of variance was performed toest differences in labile P forms in different management mod-ls, irrigation regimes and soil layers. For each of the soil layers,wo factor analysis of variance was carried out using the GLMrocedure. To achieve normality and homogeneity of variance inhe data, proportions were transformed by the arcsin square rootrior to the statistical analyses. The differences in regressionines within rainfed or irrigated soils were tested using one-waynalysis of covariance, which included the interaction betweenhe factor and the covariable. In order to analyse non linearelationships, some of the data were transformed using naturalogarithms.

. Results

.1. Labile inorganic and organic P (NaHCO3-Pi andaHCO3-Po)

Labile inorganic P (NaHCO3-Pi) was highest in the soilurface layer in all treatments (Fig. 1). Fields with irrigationhowed a sharp labile Pi decrease with soil depth. In rainfedelds, the decrease in labile Pi with depth was much more grad-al, especially in organic rainfed plots, which hardly showed anyecrease. Interestingly, after 18 years of organic management,he levels of Pi in the organic fields were different to the levelsn conventional systems that had received mineral P fertilisers.he differences, however, depended on the amount of manurepplied (four times more in irrigated than in rainfed soils), andn the soil depth (see the interactions in Fig. 1). The majorifferences in NaHCO3-Pi were observed in the surface layer.rganic irrigated fields (that had received 10 Mg ha−1 yr−1 ofanure) showed the highest Pi content while organic rainfedelds (that had received 5 Mg ha−1 of manure in alternateears) showed the lowest values. Nevertheless, the absoluteowest values for labile P were obtained in the 20–30 cmayer in the conventional irrigated plots. Like NaHCO3-Pi,aHCO3-Po decreased with soil depth in irrigated plots, while

n rainfed fields it remained rather constant throughout the soilrofile (Fig. 1). Unlike NaHCO3-Pi, NaHCO3-Po decreasedn all organically managed soils regardless of the fertilisationegime.

wr

r

aHCO3-Pi (Lopez and Lopez, 1978). Bars refer to the standard error of theean.

.2. The relationships between NaHCO3-P and soilroperties

The slight increase in NaHCO3-Pi in the surface layerbserved in the organically managed irrigated soils coincidedith increases in organic matter and decreases in both car-onates and the pH of this layer (Table 1). However, in therrigated soils, we did not observe any relationship betweenoil carbonates and NaHCO3-Pi or NaHCO3-Po (Fig. 2), whileaHCO3-Pi showed a negative relationship with soil pH. In irri-ated soils, NaHCO3-Po only showed this negative relationshipnder organic management. In contrast, both the NaHCO3-Pind NaHCO3-Po of all irrigated soils showed a strong positiveelationship with soil organic C and N, irrespective of the typef fertiliser (organic or mineral) applied (Fig. 3).

NaHCO3-Pi and NaHCO3-Po in rainfed conventional soilshowed a strong negative relationship with both soil pH andarbonate content. In contrast with other soils, organically man-ged rainfed soils showed a positive effect of carbonates thatas highly significant for NaHCO3-Pi and was also observed

or NaHCO3-Po. Likewise, the effects of increasing pH onaHCO3-Pi availability were largely reduced in low P, organ-

cally managed rainfed soils. The slope of the regression lineas much lower in this case (Fig. 2). It is noteworthy that this

eduction was not observed for NaHCO3-Po.As in irrigated soils, NaHCO3-Pi and NaHCO3-Po P in

ainfed conventional soils showed strong positive relationships

J. Romanya, P. Rovira / Europ. J. Agronomy 27 (2007) 62–71 65

Table 1Soil characteristics in the studied treatments

Soil depth (cm) Irrigated Rainfed ANOVA, p

Conventional Organic Conventional Organic Manage. Irrig. Manage.× Irrig.

0–10 C (%) 0.89 (0.12) 1.26 (0.11) 0.91 (0.06) 0.79 (0.03) n.s. 0.019 0.005N (mg g−1) 1.02 (0.10) 1.30 (0.12) 1.05 (0.06) 0.85 (0.05) n.s. 0.023 0.008C/N 8.45 (0.42) 9.83 (0.50) 8.69 (0.28) 9.56 (0.59) 0.019 n.s. n.s.Carbonates (%) 32.69 (1.73) 24.01 (0.94) 28.18 (1.17) 30.14 (1.14) 0.016 n.s. 0.000pH 8.82 (0.08) 8.41 (0.07) 8.52 (0.05) 8.56 (0.05) 0.005 n.s. 0.001Clay (%) 23.82 (1.88) 30.53 (2.88) 29.58 (3.20) 30.60 (0.84) n.s. n.s. n.s.

10-20 C (%) 0.69 (0.06) 0.86 (0.07) 0.85 (0.06 0.72 (0.03) n.s. n.s. 0.013N (mg g−1) 0.88 (0.08) 0.95 (0.07) 0.96 (0.06) 0.78 (0.04) n.s. n.s. 0.060C/N 7.93 (0.49) 9.38 (0.78) 9.14 (0.78 9.18 (0.35) n.s. n.s. n.s.Carbonates (%) 32.81 (1.82) 24.43 (1.10) 28.29 (1.30) 29.75 (1.12) 0.012 0.000 0.001pH 8.92 (0.05) 8.58 (0.06) 8.67 (0.05) 8.72 (0.03) 0.006 n.s. 0.000Clay (%) 24.93 (2.09) 28.91 (2.22) 32.52 (1.09) 30.72 (3.02) n.s. 0.053 n.s.

20–30 C (%) 0.52 (0.04) 0.65 (0.08) 0.68 (0.05) 0.65 (0.04) n.s. 0.050 n.s.N (mg g−1) 0.61 (0.05) 0.78 (0.08) 0.90 (0.05) 0.78 (0.04) n.s. 0.018 0.015C/N 8.82 (0.70) 8.27 (0.59) 7.99 (0.40) 8.64 (0.64) n.s. n.s. n.s.Carbonates (%) 34.54 (2.58) 24.61 (1.12) 28.54 (1.52) 29.83 (1.24) 0.020 n.s. 0.003pH 9.00 (0.05) 8.57 (0.08) 8.77 (0.06) 8.76 (0.04) 0.001 n.s. 0.002Clay (%) 26.70 (2.90) 28.96 (1.69) 29.63 (1.43) 30.20 (2.30) n.s. n.s. n.s.

Standard error is shown in parenthesis italics. n.s. refers to non significant (p < 0.050).

Table 2Pearson correlation coefficients between soil organic C and N, carbonates and pH, calculated from irrigated and rainfed treatments including both organically andconventionally managed soils and for all soil layers (n = 72)

Organic C (%) N (%) Carbonates (%) pH

Rainfed soilsOrganic C (%) 1 0.678 (p = 0.000) −0.315 (p = 0.007) −0.406 (p = 0.000)N (%) 1 −0.536 (p = 0.000) −0.339 (p = 0.004)Carbonates (%) 1 n.s.

Irrigated soilsOrganic C (%) 1 0.887 (p = 0.000) −0.370 (p = 0.002) −0.559 (p = 0.000)

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.s. refer to non significant.

ith organic C and N (Fig. 3). An analysis of the organicallyanaged rainfed treatment revealed that, in this case, high soil

rganic C and N content did not correspond with increasesn any of the NaHCO3-P forms. Except for the relationshipetween NaHCO3-Po and organic C, the covariance analysesor rainfed soils showed that there was significant interaction

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able 3tep-wise multiple regression models between NaHCO3-Pi, NaHCO3-Po and soil pH

reatment Dependent variable Multiple regre

rrigated conventional NaHCO3-Pi −1.41 (1.22) +NaHCO3-Po −6.93 (2.16) +

rrigated organic NaHCO3-Pi −32.01 (11.92NaHCO3-Po 13.7 (12.55) +

ainfed conventional NaHCO3-Pi 39.87 (10.85)NaHCO3-Po −2.83 (1.15) +

ainfed organic NaHCO3-Pi −69.87 (27.63NaHCO3-Po −244.74 (54.9

he standard error on the coefficients is shown in parenthesis.

−0.413 (p = 0.000) −0.528 (p = 0.000)1 0.701 (p = 0.000)

etween the management model and the covariable (organicor N). Both organic C and N negatively correlated with

oil carbonates and pH (Table 2). This indicates that the soilsith a higher amount of organic matter coincided with soilsith lower pH and with less carbonates. Multiple regression

nalyses of organically managed rainfed soils showed that

and carbon, nitrogen and carbonate content

ssion equation R2

10.23 (2.26) ln C 0.37665.99 (12.1) ln N + 0.17 (0.05) carbonates 0.510

) + 7.11 (1.34) C + 9.26 (3.51) ln carbonates 0.4764.02 (0.97) C + 7.81 (0.1) ln carbonates − 4.59 (1.47) pH 0.610

+ 65.32 (11.04) N − 4.84 (1.18) pH 0.80460.87 (11.53) N 0.458

) + 29.32 (11.65) ln carbonates − 0.91 (0.4) carbonates 0.3651) + 102.70 (23.15) ln carbonates − 3.37 (0.79) carbonates 0.436

66 J. Romanya, P. Rovira / Europ. J. Agronomy 27 (2007) 62–71

Fig. 2. Relationships between labile P forms, either organic or inorganic, and soil carbonate content and pH in conventionally and organically managed soils bothin rainfed and irrigated conditions. Regression lines are shown when significant (solid lines p < 0.05 and dashed lines p < 0.1), either for one management model ort atment

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hroughout the data set when no significant differences were found between trehe ANCOVA are indicated.

oth NaHCO3-Pi and NaHCO3-Po were only related to soilarbonate content. In the other treatments, both labile P formsere related primarily to soil organic matter (organic C and N)

nd to a lesser extent to soil carbonate content and pH (Table 3).

.3. The relationships between NaHCO3-Pi andaHCO3-Po

In Fig. 4, we show the relationships between NaHCO3-Pi

nd NaHCO3-Po across all treatments and soil layers. In rainfedoils fertilized with inorganic P, increases in NaHCO3-Pi alwaysesulted in increases in NaHCO-Po, in all layers. However, inhe rainfed organically managed soils, these relationships were

dm

N

ts. The significant factor (management model), covariables and interactions in

nly observed for the 20–30 cm layer, which had a much lowerlope than the conventional soils. As a result of the low valuesf NaHCO3-Pi observed in the organically managed 0–10 and0–20 cm soil layers, we did not find significant relationshipsetween NaHCO3-Pi and NaHCO3-Po. In spite of this, covari-nce analyses showed statistical differences in the slopes ofonventional and organically managed treatments in all studiedoil layers. In rainfed soils, there were only slight variations inaHCO3-Po among the different management models and soil

epths (Fig. 1). In rainfed soils, NaHCO3-Pi depletion occurredainly in the surface layer.In irrigated soils, the relationships between NaHCO3-Pi and

aHCO3-Po showed a different trend to that in rainfed soils

J. Romanya, P. Rovira / Europ. J. Agronomy 27 (2007) 62–71 67

Fig. 3. Relationships between labile P forms, either organic or inorganic, and soil organic C and N in conventionally and organically managed soils both in rainfeda s p <t signifia

(alimgtomsfail

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nd irrigated conditions. Regression lines are shown when significant (solid linehe data set when no significant differences were found between treatments. There indicated.

Fig. 4). In this case, the relationships between NaHCO3-Pind NaHCO3-Po were very different depending on the soilayer. Like rainfed soils, the 0–10 cm layer showed a pos-tive relationship in both treatments. However, organically

anaged soils had slightly higher levels of NaHCO3-Pi for aiven level of NaHCO3-Po. In the irrigated 10–20 cm layer,he relationship between NaHCO3-Po and NaHCO3-Pi wasnly significant for the conventional soils and the slope wasuch lower. In the 20–30 cm layer, conventionally managed

oils showed a strong relationship between both labile P

orms. In all cases, the lack of relationships (low slopesnd/or non significant relationships) between organic andnorganic labile P forms occurred when NaHCO3-Pi was veryow.

sse(

0.05 and dashed lines p < 0.1), either for one management model or throughoutcant factor (management model), covariables and interactions in the ANCOVA

. Discussion

.1. Labile inorganic P

The NaHCO3-Pi values obtained in our soils are in the low-st range when compared with other agricultural calcareous soilsYang and Jacobsen, 1990). Diagnostic norms for the Olsen Pest for extensive Spanish grassy crops growing on clay loamsndicated that irrigated and rainfed conventional soils (0–20 cm)howed low P availability, while organically managed rainfed

oils showed very low P availability (Lopez and Lopez, 1978;ee the very low reference in Fig. 1). These very low labile P lev-ls could be associated with the low levels of manure applicationabout 2.5 Mg of poultry manure ha−1 yr−1) that these soils have

68 J. Romanya, P. Rovira / Europ. J. Agronomy 27 (2007) 62–71

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ig. 4. Relationships between NaHCO3-Po and NaHCO3-Pi for soil treatmentssolid lines p < 0.05 and dashed lines p < 0.1). The significant factor (manageme

eceived in the last 18 years. A tentative P budget in the rainfedrops could help interpreting this fact; in the studied area underainfed conditions yield ranges from 2 to 4 Mg ha−1 yr−1inrganically managed crops and from 3 to 6 Mg ha−1 yr−1inonventional crops. This represents a P demand between 9.5nd 19 kg P ha−1 yr−1 for the organically managed crops andetween 14 and 28 kg P ha−1 yr−1 for the conventional in whichnorganic P input (30–40 kg P ha−1 yr−1) is well above therop demand. In contrast, in organically managed crops totalinput (organic + inorganic; 15–20 kg P ha−1 yr−1) falls within

he range of P demand. Under these conditions, considering theigh P retention calcareous soils and the fact that the organic Pdded with manures is not readily available to plants we sug-est that in rainfed organically managed plots P additions mayot match crop demand. Other studies in maize and in cerealrops have shown that NaHCO3-Pi decreased when little or nowas added to the soil, either in organic or inorganic forms

Schmidt et al., 1996; Zhang and MacKenzie, 1997; Kuo et al.,005). Some recent extensive studies on organic arable farm-ng systems have also pointed out a decrease in P availability inrganic arable systems, which was attributed to the use of soilreserves (Gosling and Shepherd, 2005; Wivstad et al., 2005).

n our study as organically managed soils were conventionallyanaged prior to its reconversion to organic management, 18

ears before sampling, we can assume that the low NaHCO3-i compared to conventional soils under rainfed conditions wasssociated to the different fertilisation regime that they wereubmitted during the organic farming period.

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yers (0–10, 10–20 and 20–30 cm). Regression lines are shown when significantdel), covariables and interactions in the ANCOVA are indicated.

A completely different situation was found in our organ-cally managed irrigated crops that received even largermounts of P (60–80 kg P ha−1 yr−1) than the conventionalrops (50–60 kg P ha−1 yr−1). Considering that yield in irri-ated crops is 7 Mg ha−1 yr−1 for organically managed cropsnd 9–10 Mg ha−1 yr−1 for the conventional crops we can thenalculate that P demand in organically managed soils is clearlyower than P inputs (40 kg P ha−1 yr−1) while in conventionallots it falls within the range (51–57 kg P ha−1 yr−1). Likelys a result of this fact the accumulation of both organic andnorganic labile P in the soil surface is somewhat minor in the irri-ated conventionally managed plots as compared to the irrigatedrganically managed soils. Differences in soil tilling operationsconventionally managed crops are normally ploughed with aouldboard while organically managed plots are ripped down

o 20 or 50 cm using a chisel plough) may have also contributedo this fact.

.2. Labile P forms, carbonates and soil pH

In calcareous soils, inorganic P solubility is highly relatedo carbonate chemistry (Pena and Torrent, 1990). Under theseonditions, a fraction of the large amount of P that is boundo Ca (Carreira et al., 2006) may be released with a decrease in

oil pH. Negative relationships between NaHCO3-Pi and soil pHave been observed by other authors after fertiliser applicationTiessen et al., 1984). However, these authors used a datasetith a wider and lower range of soil pH than ours. Despite

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orking with a fairly narrow range of pH, in all our soils weound negative relationships between NaHCO3-Pi and soil pH.hese relationships were highly significant for conventionallyanaged rainfed soils. In contrast, in low P, rainfed organi-

ally managed soils we found a weaker relationship betweenaHCO3-Pi and soil pH, with a much lower slope than in con-entionally managed soils (Fig. 2). This suggests that labile Pools were less related to changes in soil pH in soils with a lowaHCO3-Pi. The relationships between NaHCO3-Pi and soil pH

n the irrigated soils were weaker, and there were no differencesetween the management practices tested.

In irrigated soils, the carbonate content was not related toaHCO3-Pi. This was true in spite of the differences in carbon-

te content observed between organically and conventionallyanaged soils (Table 1). In contrast, carbonates were highly

elated to NaHCO3-Pi in rainfed soils. The sign of the slope ofhe relationship depended on the fertilisation regime (Fig. 2).ecreases in NaHCO3-Pi with decreasing carbonates have beenbserved by other authors (Sharpley and Smith, 1985). This rela-ionship can be interpreted as increased geochemical protectionf labile P, occurring in soils with a high carbonate content.he opposite trend observed in our rainfed organically managedoils suggests that the mechanisms of geochemical protection ofmay be altered in soils that have received little P input over

he years.The relationship among pH, carbonates and NaHCO3-Po was

ot as clear as that of NaHCO3-Pi. Turner et al. (2003), workingith North American semiarid arable soils covering a wide rangef carbonate contents and a wider and lower pH range than oursfrom 5.2 to 8.2), found a strong negative relationship betweenaHCO3-Po and pH and no relationship between NaHCO3-Po

nd carbonates. Similarly, in our data set, NaHCO3-Po showedstronger relationship with pH than with carbonate content.his suggests that changes in the mobility of NaHCO3-Po areostly regulated by pH. In contrast, in rainfed organically man-

ged soils, NaHCO3-Po showed a mild positive relationship witharbonates. This suggests that, despite the high pH and carbon-te content ranges of our dataset, soils with a high content ofarbonates may have shown increased protection of this fractionrom mineralisation. In conventionally managed soils NaHCO3-o forms did not seem to be protected by high levels of soilarbonates.

.3. Mechanisms of regulation of labile P forms

Multiple regression analyses indicated that labile P forms inainfed organically managed soils were primarily regulated byoil carbonate content. Indeed, this was the only case in which thevailability of P seemed to be independent from C and N cycling.he independence between decomposition processes and soil Pvailability has been discussed by many authors studying nat-ral soils (McGill and Cole, 1981; Tate and Salcedo, 1988). Aigh abundance of Ca-phosphates in calcareous soils may fur-

her enhance this independence (Lajtha and Schlesinger, 1988).n contrast, in our irrigated and conventionally managed soils,hich had higher P availability than organically managed soils,aHCO3-P forms were primarily regulated by soil organic mater

pioa

Agronomy 27 (2007) 62–71 69

ontent (total organic C or N) and in some cases, carbonates andH showed secondary effects (Table 3).

It has been shown that additions of soil organic mattermprove P fertility by decreasing the P sorption capacity, increas-ng labile P fractions and promoting the accretion of organic PHaynes and Mokolobate, 2001; Reddy et al., 2005). In calcare-us soils, it has also been shown that soil pH mechanisticallyelates to soil pCO2 (Bruckert and Rouiller, 1987) and that soilrganic matter inputs can also enhance soil biological activity. Inur data set, soils with high C and N content coincided with lowarbonates and pH (Table 2). The combination of these proper-ies may have enhanced microbial activity and thus P mobility.n contrast, soils with low organic C and N, high carbonate con-ent and pH probably presented lower microbial activity and P

obility.In rainfed soils, the variation of NaHCO3-Pi associated

ith soil properties depended on the last 18 year’s fertilisa-ion regime. Differences between management practices wereighest in soils with high organic C and N and low pH andarbonates. These differences were reduced in soils with lowrganic C and N and high pH and carbonates. Therefore, itppears that rainfed organically managed soils that had beenertilised at low rates for 18 years, showed the greatest deple-ion of NaHCO3-Pi in soils with high organic C and N and lowH and carbonates. The differences in NaHCO3-Po betweenainfed management practices were less clear. In spite of theifferences in soil properties (organic C and N, carbonates andH) observed between management models in the irrigated soilsTable 1), these soils did not show any qualitative changes in Pynamics associated with the fertilisation regime (organic orineral).

.4. Labile organic P and labile inorganic P relationships

Several field studies in acid soils have found that P fertilisa-ion increases NaHCO3-Pi and reduces NaHCO3-Po (Zhang and

acKenzie, 1997; Kuo et al., 2005). This suggests that P fertil-sation enhances the mineralisation of NaHCO3-Po. However,hang and MacKenzie (1997) found increases in both labile P

orms when very high amounts of P were added to the soils andaHCO3-Pi was well above 180 mg kg−1. This suggests thataHCO3-Po may build up under high P availability. Schmidt

t al. (1996) found increases in NaOH-Po at much lower Pdditions (40 kg P ha−1) in moderately acidic soils. Our calcare-us soils, which had levels of NaHCO3-Pi much lower than theforementioned acid soils (less than 10 mg kg−1), never showedny reduction of NaHCO3-Po when NaHCO3-Pi increasedFig. 4). In our soils, in situations with moderate P avail-bility, we found increases of NaHCO3-Po with NaHCO3-Pi.n conditions of very low P availability, in which NaHCO3-i had been largely depleted, NaHCO3-Po remained almostnchanged when NaHCO3-Pi increased. Therefore, it appearshat NaHCO3–Po was not a significant source of NaHCO3-Pi,

articularly in situations of P starvation. Guo et al. (2000), work-ng in a range of tropical soils that had received high amountsf inorganic P, concluded that organic P fractions (NaHCO3-Pond NaOH-Po) were not significant contributors to available P.

7 p. J.

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ther authors have found that labile organic P (NaHCO3-Po)nly contributes slightly to available P in acid soils (Beck andanchez, 1994; Zhang and MacKenzie, 1997). Kuo et al. (2005),orking on a series of temperate acid soils, suggested that the

ontribution of organic P to available P was important only whenhe ratio of NaOH to NaHCO3-Po was high (over 4), indicatinghat the source of available P is mainly the NaOH-Po fraction.t is interesting to point out that in the tropical soils databasef Guo et al. (2000), the NaOH-Po to NaHCO3-Po ratio was inost cases well below 4. In soils with a high carbonate content

nd low organic C this ratio was even lower than one. Thereforet appears that in tropical soils, organic P forms extracted byaOH or NaHCO3 cannot greatly contribute to P availability

pecially in soils with high carbonate content and low organic. We did not analyse NaOH-Po in our semiarid soils. However,e did analyse NaOH-Pi in a subset of samples and it was onlyne-third of NaHCO3-Pi (data not shown).

The first soil layer of our organically managed irrigatedlots showed that for a given level of NaHCO3-Po, the levelf NaHCO3-Pi was slightly higher than in conventionally man-ged plots (see 0–10 cm, Fig. 4). As these soils received largermounts of P, partly in organic forms, than conventionally man-ged irrigated soils, this increase in Pi can be attributed to theigher mineralisation of P occurring in organically managedoils with a higher organic P input and organic matter con-ent (see Table 1). Indeed, the increases in NaHCO3-Pi in therganically managed irrigated plots (first layer) relative to theonventional soils are of the same order of magnitude as theecreases in NaHCO3-Po (see Fig. 1).

As a general trend we observed that in soils with moder-te P availability increases in NaHCO3-Po resulted in increasesf NaHCO3-Pi (see rainfed conventionally managed soils andrrigated soils at the soils surface; Fig. 4). In contrast, in soilsith very low P availability, the NaHCO3-Po stays nearly con-

tant during the depletion of NaHCO3-Pi (see rainfed organicallyanaged soils and 20–30 cm irrigated conventional soils; Fig. 4).his suggests that under conditions of severe P starvation theaHCO3-Po pool is not available for mineralisation and thus it

annot contribute to P availability.

.5. P availability and crop P uptake

The soil layers with the lowest level of NaHCO3-Pi did notoincide among treatments. In rainfed soils, NaHCO3-Pi wasow in the 0–10 and 10–20 cm layers for the organically man-ged soils. In irrigated soils, low NaHCO3-Pi levels occurredainly in the subsurface soil layers (10–20 and 20–30 cm) and

t was especially significant in the conventional soils in the0–30 cm layer. It appears, therefore, that in crops growing inemiarid conditions under irrigation a significant part of P cropptake occurs in the subsurface layers. In contrast, under rain-ed conditions, P crop uptake mainly occurs in the soil surface.ncreased soil organic C and decreased pH and carbonate con-

ent in irrigated organically managed soils may have favoured

availability in the 20–30 cm in comparison with conventionaloils. This suggests that organic management of irrigated soilsncreases P availability down to 30 cm, while in irrigated con-

A

Agronomy 27 (2007) 62–71

entional soils the effects of mineral P addition are concentratedn the first 10 cm of soil.

. Conclusions

In low organic matter agricultural calcareous soils the rela-ionships between pools of NaHCO3-Pi and soil propertiesepended on the past soil fertilisation regime. In soils that haveeceived high P inputs over the last decades, NaHCO3-Pi mobil-sation is primarily related to soil organic matter dynamics. Inoils that have received low P inputs, P mobilisation is mainlyelated to the geochemical equilibriums associated with the soilarbonate content and pH. High amounts of Ca phosphate mayecome completely unavailable in soils with low organic matterontent, while they can be mobilised by decreases in soil pHssociated with soil biological activity. Thus in conditions ofstarvation, high NaHCO3-Pi depletion mainly occurs in soilsith high organic C and N and decreased pH.NaHCO3-Po in calcareous soils is generally less mobile than

aHCO3-Pi. NaHCO3-Po contribution to P availability appearso be relevant in soils with moderate P availability that haveeen organically fertilized. In contrast, in P depleted soils,aHCO3-Po does not contribute to P availability, especiallyhen carbonate content and pH are high and organic C andare low.In conditions of moderate P availability, labile P cycling does

ot appear to be independent from C and N cycling. In contrast,n conditions of P starvation, P cycling decouples from otherutrients, especially in soils with higher organic Carbon content.n calcareous soils, management practices need to increase oraintain the levels of soil organic Carbon in order to mobilise

. In organically managed soils, P inputs and outputs need to bealanced on a mid to long-term basis.

cknowledgements

This work was made possible thanks to an agreement betweenhe Environmental Assessment Section of the Spanish Ministryf the Environment and the University of Barcelona from 2003o 2004 and thanks to the project AGROECO (CGL2006-13190-O3-01) of the Spanish Ministry of Science and Technology. Weish to thank Olga Carreton, Luis Lopez-Sangil, Jordi Torras,ontse Toribio and Eva Bertran for their help in the field and

ab work. Finally, we also wish to thank the helpful commentsf one of the referees.

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