0038-075X/05/17010-822-831 October 2Soil Science Vol. 170. NoCopyright Q 2005 by Lippincott Williams & Wilkins, Inc- Printed in US

SOIL PHYSICAL PROPERTIES AND AGGREGATE-ASSOCIATEDC, N, AND P DISTRIBUTIONS IN ORGANIC AND

CONVENTIONAL CROPPING SYSTEMS

V. Steven Green1, Michel A. Cavigelli1, Thanh H. Dao 2, and Dennis C. Flanagan 3

Organic farming, which is growing in popularity, has been proposedas a sustainable alternative to conventional farming practices. However,it is not known how organic farming systems affect soil erosion risk andsediment-bound nutrient transport. Our objectives were to compare soilerosion risk and sediment bound nutrient transport potential for grain-based conventional and organic cropping systems by. determiningselected soil physical properties and distributions of carbon (C), nitrogen(N), and phosphorus (P) in soil aggregates of the 0-5 cm depth of aChristiana-Matapeake-Keyport soil association. We measured soil bulkdensity, aggregate stability, aggregate size distributions, and total C, N,and P associated with five soil aggregate size classes in no-till (NT) andchisel till (CT) systems and in an organic system (ORG). No-till soils hadlesser bulk density and greater aggregate stability than did CT and ORGsoils. Carbon, N, and P concentrations were greater in large (>2.00 mm)and small macroaggregates (0.21 to 2.00 mm) than in microaggregates(<0.21 mm) regardless of cropping system. When nutrient concentrei-tions were combined with aggregate distribution data, the quantity ofaggregate associated nutrients was greatest in microaggregates in ORGand CT soils but greatest in macroaggregates in NT soils. These resultsindicate an increased risk of sediment associated nutrient transport fromORG and CT soils compared with NT soils, since microaggregates inthese soils are preferentially lost through sediment transport. The NTcropping system promoted macroaggregate formation and reduced therisk of particulate nutrient transport in this warm, humid region soil.(Soil Science 2005;170:822-831)

Key words: Organic farming, no-till, sustainable agriculture, soilaggregates, tillage, nutrient erosion.

N the Chesapeake Bay watershed, as in manyparts of the world, there is considerable in-

terest in developing agricultural managementsystems that improve soil quality and minimize

'United States Department of Agriculture, Agricultural Research Service,

Sustainable Agricultural Systems Laboratory, Building 001 BARC-West, 10300Baltimore Ave., Beltsville, MD 20705-2350. Dr. Green is corresponding

author. E-mail: greenst@a.ars.usda.gov2United States Department of Agriculture, Agricultural Research Service,

Environmental Management and By-product Utilization Laboratory, Building

306 BARC-East, 10300 Baltimore Ave., Beltswille, MD 20705-2350.3United States Department of Agnculture, Agricultural Research Service, National

Soil Erosion Research Laboratory, 275 South Russell St., West Lafayette, IN

47907-2077.

Received Jan. 10, 2005; accepted May 16, 2005.

DOL: 10.1097/01.is.0000190509.18428.le

off-farm water quality impacts (Boesch et al.,2001). Sediment delivery from erosion is one ofthree major mechanisms by which agriculturalsources contribute to water quality problems(Boesch et al., 2001). Eroded sediments cancarry sorbed nutrients to surface water bodiesthat can subsequently desorb and impair waterquality (Uusitalo et al., 2001; Zhang et al., 2003).Excess N, P, and organic matter in surface waterbodies causes eutrophication and subsequentdepletion of oxygen (Boesch et al., 2001). Theresulting anoxic or hypoxic conditions canimpair aquatic habitat and adversely affectaquatic organisms (Boesch et al., 2001). Addi-tionally, nutrients eroded from farm fields areno longer available to the crops and pose a di-rect economic loss to the farmer.

822

00510

.A.

AGGREGATE ASSOCIATED C, N, AND .P

Agricultural management influences aggre-gate size distributions. On temperate region soils,NT practices have been shown to increase thewater stable aggregate fraction and maintainaggregates of a larger size than in conventionallytilled soils (Beare et al., 1994b; Six et al., 2000).No-till practices allow continued aggregationover a long period of time, whereas conventionaltillage disrupts the aggregation process regularly.Using cover crops (Sainju et al., 2003) and ani-mal manures (Mikha and Rice, 2004), two com-mon practices in organic farming, have also beenshown to increase soil C and improve soilaggregation.

Agricultural management can also influencenutrient concentrations in different aggregatesize classes. Various researchers have found thatnutrient concentrations are greater in macro-aggregates than in microaggregates in soils fromtemperate climates (Bhatnagar and Miller, 1985;Bhatnagar et al., 1985; Elliott, 1986; Jastrowet al., 1996), whereas others have found greaternutrient concentrations in microaggregatesthan in macroaggregates (Elliott et al., 1991;Jastrow et al., 1996; Linquist et al., 1997; Wanand EI-Swaify, 1998). These discrepancies canbe explained, in part, by differences in land useand management. Jastrow et al. (1996) foundthat macroaggregates were enriched in soilorganic C (SOC) in pasture soils, but micro-aggregates were enriched in SOC in continuouscorn soils of the same soil type (Mollisol) andseries. Bhatnagar and Miller (1985) found thatlarge aggregates (>2.0 mm) contained more ex-tractable and total P than smaller ones (<1.0mm) when manure was applied, but when in-organic P was applied, no differences wereobserved. They concluded that P was preferen-tially absorbed by large aggregates when liquidpoultry manure slurry was applied. Mikha andRice (2004) found that tillage and nutrientsource greatly affected aggregate associated total"C arid N: macroaggregates were enriched in"C and N under no-till and/or manure applica-tion compared with under chisel tillage and/orno manure application.

Although considerable research has beenconducted on soil structure and soil erosion inno-till and chisel till cropping systems, thereare few studies on the risk of soil and nutrientloss in organic farming systems. While organicfanning is often assumed to increase soil erosioncompared with conventional tilled systems dueto the increased amount of tillage often requiredfor weed control (Pacini et al., 2004), organic

systems also usually involve relatively greaterorganic matter inputs as plant and animal man-ures, which have been shown to increase soilstructural stability (Mikha and Rice, 2004; Sainjuet al., 2003; Tisdall and Oades, 1979; Whalenet al., 2003). The net effect of increased tillageand increased organic matter inputs in organicsystems remains unknown. Since organic fanningis growing in popularity in many areas of theUnited States and Europe, it is important todetermine the effect organic systems have onsoil and nutrient erosion. Two studies that havecompared soil erosion in organic and convention-al farming systems suggest that organic wheatsystems that include cover crops (Reganold et al.,1987) and complex organic rotations that includeperennial crops (Siegrist et al., 1998) have lowersoil erosion than conventional systems. Organicfarming systems, due to increased organic matterinputs, have also been shown to have greater soilorganic matter content and/or water stableaggregation-properties positively correlatedwith reduced soil erosion (Guerra, 1994; Ladoet al., 2004)--compared with tilled, conventionalsystems (Drinkwater et al., 1998; Miider et al.,2002; Reganold et al., 1987; Robertson et al.,2000; Siegrist et al., 1998). All of these studieswere conducted on Alfisols or Mollisols in coolerregions and only one study includes a comparisonbetween an organic system and a no-till system(Robertson et al., 2000). There are no studiescomparing organic and conventional croppingsystems on more fragile soils in warm, humidclimates.

A better understanding of the effects ofdifferent cropping systems on soil physicalproperties and nutrient distributions in Ultisolswill enable a more realistic evaluation of erosionpotential and environmental sustainability ofgrain-based organic and conventional croppingsystems on these soils. Surface soil is selectivelyeroded from agricultural lands, with smallerparticles and aggregates eroding preferentially(Foster et al., 1985). The potential for waterquality problems associated with sediment deliv-ery can be estimated by measuring the amountof soil nutrients associated with various particleand aggregate size classes in surface soils (Elliottet al., 1991). The objectives of this study wereto determine the effects of no-till (NT), chiseltill (CT) and organic (ORG) cropping systemson Ultisols on (i) aggregate stability and sizedistribution and (ii) the distribution of C, N,and P among different aggregate size classes. Wehypothesized that (i) NT will have a greater

VOL. 170 - No. 10 823

GREEN, ET AL.

proportion of large and small macroaggregatesthan CT and ORG due to lack of tillage in NT,and ORG will have a greater proportion of largeand small macroaggregates than CT due togreater organic matter inputs in ORG than inCT; and (ii) for these same reasons, the stabilityof aggregates will also be in the orderNT>ORG>CT. Thus, we hypothesize that thepotential for nutrient loss by erosion will begreatest in CT, least in NT and intermediatein ORG.

MATERIALS AND METHODS

Study Site

The 16-ha study site is at the western edgeof the Atlantic coastal plain near Beltsville,Maryland. The dominant soil types are Chris-tiana (fine, kaolinitic, mesic Typic Paleudults),Matapeake (fine-silty, mixed, semiactive, mesicTypic Hapludults), and Keyport (fine, mixed,semiactive, mesic, Aquic Hapludults). Slopesacross the site vary from 0 to 8%. From 1985to 1995, the entire 16 ha site was managed asone field under permanent no-till management.From 1985 to 1992, the field was planted toalfalfa (Medicago sativa L.) and dairy manure wasapplied regularly. From 1993 to 1995, the fieldwas planted to corn (Zea mays L.). The study sitereceives an average of 1110 mm of precipitationannually, distributed evenly through the year.

Cropping Systems and Cultural Practices

In 1996, the Beltsville Farming SystemsProject (FSP), a long-term study of croppingsystems was established at the site (www.ba.ars.usda.gov/sasl/research/fsp.html; verifiedDecember 10, 2004). The FSP consists of fivecropping systems arranged in four randomizedblocks. We report on data collected from thecorn phase of the rotation for three of thecropping systems: 1) a CT- and' 2) an NT-based 3-year corn-soybean [Glycine max (L.)Merr.]-wheat (Triticum aestiwma L.)/soybean

rotation, and 3) a 3-year ORG corn-soybean-wheat/hairy vetch (Vicia villosa Roth.) rotation(Table 1).

The NT treatment had not been tilled for atleast 18 years before soil sampling. The CTtreatment was chisel-tilled (- 15 cm depth),disked with a tandem disk, and field cultivatedbefore planting for both corn and soybean. Inpreparation for winter wheat, CT was fieldcultivated before planting. The ORG treatmentwas moldboard plowed and disked beforeplanting corn and soybean and disked beforeplanting wheat. The ORG treatment was alsorotary hoed 5 and 10 days after planting andcultivated 3 and 4 weeks after planting for bothcorn and soybean.

The CT and NT treatments were fertilizedwith mineral fertilizers in accordance withUniversity of Maryland soil testing recommen-dations for both corn and wheat. Typical Nfertilizer application rates were 168 kg ha- 1

starter fertilizer (10-20-10) applied at planting,and sidedress urea ammonium nitrate at -150kg N ha-1 applied approximately 1 month afteremergence for CT and NT corn. Urea ammo-nium nitrate (45 kg N ha 1) was applied to CTand NT wheat in the spring. Just before plantingwheat in the ORG treatment, approximately5.1 Mg ha-1 (dry weight) of poultry (chicken,Gallus gallus domesticus) litter was incorporatedinto the soil by disking. Corn in ORG wassupplied with N from the vetch cover crop,which was incorporated by moldboard plowing.Potassium (form approved for application inorganic systems) was also applied to ORG inaccordance with soil test results.

Weeds were controlled in each of thecropping systems using practices common tothe region and system. Chemical herbicideswere applied to NT and CT corn, soybean,and wheat according to University of Marylandweed management recommendations. Weedcontrol for ORG was accomplished by pri-mary tillage, rotary hoeing, and cultivating.

TABLE I

Cropping systems and management of the Beltsville Farming Systems Project

Cropping system Crop rotation Nutrient management Weed management Cover crop

No-till (NT) C-S-W/S* Mineral fertilizer Herbicides Rye after corn

Chisel till (CT) C-S-W/S Mineral fertilizer Herbicides Rye after cornOrganic (ORG) C-S-W/V Legume cover crop + Cover crops + Rye after corn + hairy

poultry litter cultivation vetch after wheat

*C, corn; S, soybean; W, winter Wheat; V, hairy vetch.

824 SOIL SCIENCE

AGGREGATE ASSOCIATED C. N, AND P

Soil Sampling

Soil samples were collected before corn har-vest in September 2003 from the four replicatefield plots of each cropping system. Bulk soilused for aggregate analyses was collected fromthe 0 to 5 cm depth of nonwheel traffic inter-rows by compositing four, shallow excavations(0.2 by 0.1 in by 0.05 m deep) from each plot.

Soil Physical Properties

Soil bulk density (BD) samples of the 0 to 5cm surface layer were taken from nonwheeltraffic interrows using a 10 cm diameter coringdevice according to the procedure of Arshadet al. (1996). Bulk density measurements wereused both as an indicator of soil physicalproperties and to convert soil C, N, and P datato a volume basis (Arshad et al., 1996). Wateraggregate stability (AS) of each sample was de-termined by sieving 1- to 2-num aggregates in anest of sieves for 10 minutes in deionized waterwith a 4-cm stroke length at a rate of 35 cyclesrrin-1 (Kemper and Roseneau, 1986). Sievesizes of 1.00 and 0.21 mm were used and the ASof 1- to 2- mm aggregates was calculated usingthe mean weight diameter (MWD) of aggregatescollected from each treatment as

MWD = -XiWSAi (1)

where i is the ith size class (1.00-2.00, 0.21-1.00, or 0-0.21 mm) and X is the meandiameter of each size class (based on the meanintersieve size). Water stable aggregate (WSA) isthe sand-free, water-stable aggregate mass.

Aggregate Size Distribution and Separation

Soil material collected from the field waspresieved through a 12-mm sieve. This soilmaterial was then gently sieved through a 6-rnmsieve. Soil material passing the 6-mm sieve wasair-dried, placed in polyethylene containers, andstored at 4 'C until processed.

The <6 mm, air-dry soil (30 g) was wetsieved into the following aggregate size classes:2.00 to 6.0 (large macroaggregates), 0.50 to 2.00,0.21 to 0.50 (small macroaggregates), 0.053 to0.21 (microaggregates), and <0.053 mm (silt andclay sized microaggregates and primary silt andclay particles). To separate the <0.053 mm soilfrom the water, the soil suspension was centri-fuged at 4 6 50g for 10 minutes (IEC PR-7000M,

Thermo International Equipment Company,Needham Heights, Massachusetts). The soilmaterial in each size class was then oven-driedat 65 'C for 24 hours. Aggregate size distribu-tions were calculated from the mass collectedin each aggregate size class from two replicatewet sieving iterations per sample.

Aggregate.C, N, and P Analyses

Five-gram subsamples of aggregates wereground to pass a 0.15-mm sieve, using aSampleTek Model 200 Vial Rotator (MavcoIndustries, Inc., Lincoln, Nebraska). Total Cand N were analyzed by dry combustion using aCostech ECS 4010 elemental analyzer (CostechAnalytical, Valencia, California); each samplewas analyzed in duplicate. Total P analysis of soilsamples was performed in triplicate using amodified potassium persulfate digestion proce-dure (Dao, 2004). Total P in the digestate wasdetermined by using a segmented flow, semi-automatic ion analyzer (Bran+Luebbe Inc.,Buffalo Grove, Illinois), according to the phos-phomolybdate-ascorbic acid procedure (Ameri-can Public Health Association, 1998). Wholesoil samples (before wet sieving) were alsoanalyzed for total C, N, and P according to theabove procedures. The relative risk of nutrienttransport among cropping systems was evaluatedby multiplying the aggregate size distributiondata with the nutrient distribution data to ar-rive at the proportion of each nutrient associatedwith each aggregate size class, based on thewhole soil mass (Elliott et al., 1991).

Soil Erosion Predictions

Applications of the Water Erosion PredictionProject (WEPP) model (Flanagan and Nearing,1995) were made for the site. The CLIGEN(Nicks et al., 1995) weather generator version4.3 was used to create a 100-year climate inputfile for WEPP using parameters for Washington,D.C. Soil inputs for the Matapeake silt loamsoil were obtained from the default WEPP data-bases. A 60-m-long hillslope profile with a uni-form 5% slope gradient was used as the slopeinput to the model. Detailed cropping/manage-ment rotations were created for typical CT,NT, and ORG management practices for corn-soybean-wheat/soybean rotations (includingrye cover crops after corn in all systems). TheWEPP model version 2004.7 was applied witheach of the cropping rotations, and the simulated

825VOL. 170 - No. 10

GREEN, ET AL.

TABLE 2

Whole soil physical properties and C, N, and P concentrations among cropping systems

Cropping system TC* TN TP BD AS (MWD) Macro-aggregates

kg m-3 Mg m-3 mm g g-1

NTt 32.3a- 2.8a 1.05a 1.37b 0.74a 0.62aCT 25.6b 2.3b 1.07a 1.53a 0.40b 0.35bORG 25.2b 2.3b 1.06a 1.53a 0.40b 0.31b*TC, total soil C; TN, total soil N; TP, total soil P; BID, bulk density; AS, aggregate stabilitv; MWb, mean weight diameter.tNT, no-oill; CT, chisel till; ORG, organic.

'Means followed by the same letter within a column are not significantly different at the P < 0.05 significance level.

runoff, soil loss, sediment yield, and sedimentparticle size distributions were recorded.

Data Analyses

The effects of cropping system on aggregatestability and size distribution, bulk density, andwhole soil total C, N, and P were analyzed byANOVA in a randomized complete blockdesign. The general linear model (Proc GLM)was used to conduct analyses of variance todetermine cropping system and block effects(SAS, 1999); mean separations were computedusing Duncan's multiple range test. Before dataanalysis, aggregate size distributions and nutrientproportions were subjected to an arc sinesquare-root transformation since values wereconstrained between zero and 1 (Steel et al.,1997); nontransformed means are presented.Results were considered significantly differentat the P < 0.05 level.

RESULTS

Whole Soil Physical Properties andNTntrient Concentrations

The NT soil had lesser BD and greater ASthan did CT and ORG soils (Table 2). In theNT cropping system, large and small macro-aggregates (>0.21 mm) dominated the distribu-tion, whereas in the CT and ORG systems,large and small macroaggregates representedonly a third of the total aggregates (Table 2).Aggregate stability increased as soil C increased(r2 = 0.81). The aggregate size distributions forCT and ORG soils were similar to each otherbut were different from those for the NT soil(Table 3). The greatest proportion of aggregateswas found in the 0.053 to 0.21 mm aggregatesize class in CT and ORG soils, whereasaggregates were evenly distributed among the0.50 to 2.00, 0.21 to 0.50, and 0.053 to 0.21mm size classes in NT soil. Whole soil C and N

concentrations in these cropping systems werein the order NT>CT = ORG (Table 2). TotalP was similar for NT, CT, and ORG soils(Table 2).

Nutrient Concentrations in Aggregate Size Classes

Total C, N, and P concentrations were notdistributed equally among aggregate size classes.Total C, N, and P concentrations were greatestin the 0.50 to 2.00 mm size class and least in the<0.053 mm size class across all cropping systems(Table 4). Generally, in each of the croppingsystems, the >2.00 and 0.21 to 0.50 mmaggregates had similar C, N, and P concen-trations and these were the second greatest afterthe 0.50 to 2.00 mm aggregates.

To take into account the proportion ofaggregates in any given size class, C, N, and Pconcentrations in each aggregate size class wereexpressed on a whole soil basis. Results showthat C, N, and P were generally greatest in the0.50 to 2.00 and 0.21 to 0.50 mm size classes forNT (Table 5). However, for CT and ORG, C,N, and P were concentrated in the 0.053 to 0.21nmn size class; often twice as much C, N, and Pwere in this size class than in any other size class.

TABLE 3

Aggregate size distributions from the cropping systems at

the Beltsville Farming Systems Project

Size, mm NT* CT ORG

gg

>2.00 0.1ict 0.04d 0.07c0.50-2.00 0.25a 0.1 tc 0.10c0.21-0.50 0.2 6

a 0.20b 0.13c0.053-0.21 0.23ab 0.45a 0.41a<0.053 0.15bc 0.20b 0.29b*NT, no-till; CT, chisel till; ORG, organic.

tMeans followed by the same letter within a column are not

significantly different from each other at the P < 0.05

significance level.

826 SOIL SCIENCE

AGGREGATE ASSOCIATED C, N, AND P

TABLE 4

Total C, N, and P concentration in each aggregate size class

TC* TN TPSize NTt CT ORG NT CT ORG NT CT ORG

g kg-1 -- trg kg-' -

>2.00 25.3b* 19.4ab 18.7b 2.17ab 1.74a 1.68b 809.2b 685.2b 693.Obc

0.50-2.00 27.9a 22.3a 22.3a 2.36a 1.79a 1.89a 880.8a 926.2a 826.7a

0.21-0.50 23.8b 19.1b 18.1bc 2.09b 1.61a 1.54c 788.7bc 689.8b 754.6ab

0.053-0.21 21.Oc 17.0b 16.7c 1.84c 1.48b 1.48c 732.9c 638.6b 626.4c

<0.053 13.Od 11.7c 10.7d 1.32d 1.18c 1. 11d 618.8d 605.9b 604.9c

*TC, total soil C: TN, total soil N; TP, total soil P.

tNT, no-till; CT, chisel till; ORG, organic.

TMeans followed by the same letter within a column are not significantly different at the P < 0.05 significance level.

The proportion of C, N, and P in macro-aggregates on a whole soil basis was comparedamong cropping systems. No-till had 171 and184% of the C content of CT and ORG, 173and 192% of the N content of CT and ORG,and 169 and 189% of the P content of CT andORG in macroaggregates expressed on a wholesoil mass basis (Table 5).

WEPP SimulationsThe WEPP model predicted the greatest

runoff and soil loss from the CT system and theleast from NT (Table 6). The ORG system hadsimilar predicted runoff as the NT but muchgreater soil loss (about 5 times that of NT andtwo thirds of that predicted from the CTsystem). The model-predicted average annualmicro-sediment (primary clay, primary silt, andsmall aggregates) fractions exiting the hillslopeprofile were almost identical for the three sys-tems at about 65%; predicted average annualmacro-sediment (large aggregates and primary

sand) fractions therefore were about 35% foreach cropping system. The micro-sediment waspredicted to be easily transported under any ofthe management systems for the slope conditionused (5%).

DISCUSSION

The soil physical properties among croppingsystems studied were affected mainly by tillage.Chisel till and ORG soils were similar to eachother for each of the soil physical propertiesmeasured. It appears that tillage overrides muchof the potentially beneficial soil physical prop-erty enhancement that might be expected fromthe addition of organic amendments in theORG system compared with the-CT system inthese soil types in the mid-Atlantic region.Tillage operations in CT and ORG bury thecrop residues, allowing them to decomposemore rapidly and dispersing any C and N accu-mulating benefits over a greater depth of soilthan if those residues were left on the surface

BLE 5

Proportion of C, N, and P associated with soil aggregates on a whole soil basis in NT, CT, and ORG

Size, mm NT CT ORG NT CT ORG NT CT ORG

- C/Co* N/No P/Po>2.00 0.13ct 0.05d 0.08c 0.13b 0.05d 0.08c 0.12b 0.05c 0.07d

0.50-2.00 0.31a 0.14c 0.15bc 0.29a 0.13c 0.14bc 0.28a 0.15b 0.13cd

0.21-0.50 0.27ab 0.22b 0.15bc 0.27a 0.22b 0.14bc 0.26a 0.20b 0.15c

0.053-0.21 0.21b 0.45a 0.43a 0.21a 0.44a 0.42a 0.22a 0.42a 0.38a

<0.053 0.09c 0.14c 0.19b 0.10b 0.16bc 0.22b 0.12b 0.18b 0.27b

Macro* 0.70A§ 0.41B 0.38B 0.69A '0.40B 0.36B 0.66A 0.40B 0.35B

*C/Co is the C concentration in a given aggregate size class multiplied by the proportion of that aggregate size class in whole

soil divided by whole soil C concentration; N/No and P/Po were calculated similarly.tMeans followed by the same lower case letter within a column are not significantly different at the P < 0.05 significance

level."*Macro indicates macroaggregates (>0.21 mm).

WMeans for Macro followed by the same tipper case letter within a row and element are not significantly different at the

P < 0.05 significance level.

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GIEEN, ET AL.

TABLE 6

Results of 100 year WEPP model simulations

Management Predicted runoff Predicted soil loss Micro-sediment Macro-sedimentsystem (mam y-1) (Mg ha-' yr-') exiting profile (%) exiting profile (%)

NT* 107 8.5 64 36CT 116 64 65 35ORG 108 43 65 . 35*NT, no-till; CT, chisel till; ORG, organic.

of the soil. Other studies comparing organicand conventional agricultural management haveshown an increase in water stable aggregationwith organic farming management (Mider et al.,2002; Siegrist et al., 1998). In those studies,however, dairy manure was applied annuallyand the crop rotation, whose length was morethan twice as long as in our rotations, includedthree years of a grass and clover mix. The ORGtreatment at the FSP is a 3-year rotation withpoultry litter being applied once in the 3-yearrotation. The frequency of mechanical distur-bance (e.g., primary tillage and cultivation) inORG enhances carbon mineralization andefflux (Dao, 1998; Reicosky and Lindstrom,1993) and probably negates the benefits of theadditional C input. At the FSP, we have foundgreater CO 2 efflux in ORG than CT afterspring tillage operations during the corn phaseof the rotation (unpublished data).

Soils under NT management had signifi-cantly greater AS and a greater proportion ofmacroaggregates than those under tillage (CTand ORG), as others have also found (Carter,1992; Hussain et al., 1998). Tillage reduced theproportion of macroaggregates regardless ofnutrient source (mineral fertilizer or manure).This reduction could be due to any combina-tion of three mechanisms: physical disruption ofthe aggregates, increased oxidation of organicmatter, and effects of drying and wetting inthe tilled systems (Beare et al., 1994a; Six et al.,2004). The increased C inputs of the ORGsystem may have helped maintain a similar ASas the CT system even though the ORG sys-tem receives more intensive tillage than the CTsystem.

Soil organic C has been shown to play amajor role in soil aggregation and stability intemperate soils (Franzleubbers et al., 2000;Tisdall and Oades, 1982). This is also the casewith the FSP cropping systems where we founda strong relationship between total soil C andaggregate stability (r2 = 0.81, P < 0.001). The

NT cropping system had the greatest soil Cconcentrations and the greatest AS, soil param-eters that are correlated with reduced soilerosion and reduced sediment and nutrientlosses (Alberts and Moldenhauer, 1981). Increas-ing soil organic C, in addition to enhancingbeneficial soil physical properties, improveswater holding capacity and nutrient cycling,increases biological activity (Fenton et al., 1999),and may aid in the mitigation of elevated at-mospheric CO, levels through C sequestration(Robertson et al., 2000).

We should expect the ORG system toincrease in AS only if soil C content increases.Other organic cropping systems in the UnitedStates and Europe have shown an accumulationof organic C relative to mineral fertilizer basedconventionally tilled systems. In Pennsylvania,two organic systems (beef manure + legume,and legume only as N source) accumulated soilC over a 15-year period in the surface soil(Drinkwater et al., 1998). In Michigan, wintercover crops were the only source of additionalC for organic systems that accumulated soil C inthe surface horizon over a 9-year period(Robertson et al., 2000). However, in Switzer-land, Miider et al., (2002) found only a slightincrease in organic C in an organic croppingsystem compared with a mineral fertilizersystem. In these studies, primary tillage was thesame for both organic and conventional systems.In the present study, the ORG system hadmore intensive tillage than did the CT system,but C inputs (poultry litter and hairy vetchcover crop) were greater. These additional formsof low C:N ratio organic matter may haveoxidized quickly under the warmer, more hu-mid climate of Maryland, compared with thosein Michigan, Pennsylvania, and Switzerland.

Total P typically accumulates at the surfaceof NT soils due to lack of redistribution fromtillage. The lack of differences in total soil Pamong cropping systems in the 0- to 5-cmdepth is likely due to the relatively high P

828 SOIL SCIENCE

AGGREGATE ASSOCIATED C, N, AND P

content of these soils. Soil test P (Mehlich 1) inthe Ap horizon of all plots in 1996 was -200 kgha-1, indicating similarly high total P levels inall plots. Additionally, relatively low levels of Pwere added to the plots since the trial beganin 1996. The NT and CT systems received Pin starter fertilizer only (<10 kg ha-1 y 1). Theapplication rate of P in poultry litter in theORG system was -75 kg ha-1, applied onceevery 3 years. Additional rotation cycles areneeded to determine'if the ORG system soilswill accumulate more total P than the CT andNT systems in the 'long term.

Few studies to date have examined nutrientdistributions in organic farming systems and veryfew studies include the distribution of soil Pamong aggregates (Bhatnagar and Miller, 1985;Linquist et al., 1997). Our data show anaccumulation of soil C, N, and P in macro-aggregates, which coincides with findings ofother researchers for conventional cropping sys-tems in temperate regions (Bhatnagar et al.,1985; Elliott, 1986; Jastrow et al., 1996; Mikhaand Rice, 2004; Six et al., 2000) and laboratorystudies (Bhatnagar and Miller, 1985). Althoughmanagement affected the C, N, and P concen-trations in specific aggregate size classes, the C,N, and. P distribution patterns were the sameacross management systems, with large and smallmacroaggregates having greater concentrationsthan microaggregates. These results show theimportance of stable macroaggregates in nutrientretention.

Although the C, N, and P concentrationsamong aggregate size classes indicate how theseelements are distributed among aggregate sizeclasses, they do not reflect the risk of nutrienttransport from these cropping systems. Express-ing C, N, and P on a mass basis does providethis type of assessment. A comparison of the C,N, and P content of the microaggregates amongthe cropping systems showed the NT soil tohave the least proportion of nutrients in themicroaggregate sizes (0.30 to 0.34), whereas theCT and ORG soils had approximately twicethis proportion of C, N, and P in micro-aggregates on a whole soil basis (Table 5). SinceCT and ORG had a greater proportion of C, N,and P in microaggregates than did NT, CT, andORG would be expected to have greater nu-trient losses in an erosion event than would NT.

From the WEPP simulations, we found thatmicroaggregates were more prevalent thanmacroaggregates in the eroded sediments for allcropping systems (Table 6). Since CT and ORG

soils contained a significantly greater proportionof microaggregates than did NT soils, therecould be a greater risk of nutrient transport andloss from the CT and ORG soils compared withthe NT soil, even though the nutrient concen-tration distributions among aggregate size classeswere similar in all three soils. Additionally,WEPP simulations indicated soil loss from CTand ORG to be 7.5 and 5 times the soil lossfrom NT, resulting in nutrient loss from nu-trients adsorbed to soil particles. Future model-ing studies may need to use inputs of actual fieldmeasured in situ soil particle size distributions,which could allow better evaluation of predictedsediment size classes exiting a hillslope profile.

Although cropping system did not affect C,N, and P concentration distribution patterns,cropping system did affect the proportion ofnutrients associated with different sized aggre-gates by effecting a change in the aggregate sizedistribution (Table 3). This change in aggregatesize distribution can have a profound influenceon nutrient transport risk.

Our data suggest that CT and ORG soils areat a greater risk of losing nutrients through theprocess of sediment transport than are NT soils.Although organic farming practices providedlittle in the way of soil structural enhancementsthat would reduce the risk of aggregate associ-ated nutrient transport compared with CT, theORG system maintains ground cover during agreater proportion. of the rotation cycle thandoes CT (Table 1), which would reduce itsoverall erosion susceptibility compared with theCT system (Table 6).

CONCLUSIONS

Transported sediment from agriculturalfields can be a source of C, N, and P in theenvironment. Even though large and smallmacroaggregates contained greater concentra-tions of C, N, and P than did nmicroaggregatesin this study, C, N, and P concentrations inmicroaggregates could play a major role innutrient loss due to ease of microaggregatetransport. Although NT soil has greater C, N,and P- concentrations in the top 5 cm than doCT and ORG soils, nutrient transport from NTsoils is expected to be less than from CT andORG soils, since there is a lesser proportion ofmicroaggregates in NT than in CT and ORGsoils. Cropping systems that promote macro-aggregation, such as NT, are more likely toretain C, N, and P on the field, enabling greater

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nutrient cycling and less environmental contam-ination than cropping systems with less macro-aggregation. WEPP simulations predicted thatsediment loss from NT would be 16% of thatfrom CT. There were very few differences insoil physical properties and nutrient distribu-tions between CT and ORG systems. However,WEPP simulations indicated sediment lossesfrom ORG to be 66% of that from CT.Development of a no-till or reduced till, organiccropping system may be beneficial. Additionalresearch is necessary to determine if combiningorganic farming management with no-till re-duces erosion risk compared with organic crop-ping alone while maintaining adequate yields.

ACKNOWLEDGMENTS

The authors gratefully acknowledge theassistance of Mr. George Covington and Mr.Robert Tolbert in collecting soil samples andprocessing soil aggregates. We thank Mr. GuyStone for assistance in phosphorus analysisand Dr. Brent Black for assistance in soil carbonand nitrogen analysis.

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