Soil organic matter (SOM)

One of the fundamental goals of organic farming is to increase or maintain SOM levels to improve system resilience and resistance to perturbation, enhance nutrient cycling, provide healthy and productive crops, and control pests^{7, 8,} Reference Balfour⁴⁰. Many studies confirm that organic grain cropping systems can increase soil organic carbon (SOC) and total N relative to CT systemsReference Coulter, Sheaffer, Wyse, Haar, Porter, Quiring and Klossner^32, Reference Reganold^41–Reference Liebig and Doran⁴³, regardless of whether the organic systems rely on legume cover crops alone or in combination with animal manuresReference Drinkwater, Wagoner and Sarrantonio^44, Reference Marriott and Wander⁴⁵. As noted by Leifeld et al.Reference Leifeld, Reiser and Oberholzer⁴⁶, greater SOC in organic compared with CT systems is likely due to greater C inputs rather than organic management per se.

As NT agriculture increases in prominence in the US, Brazil, Argentina, Australia and elsewhereReference Triplett and Dick¹⁵, it is important to consider NT systems when comparing ecosystem services provided by organic and conventional systems. In the US, for example, about 35% of row-crop hectares were NT planted in 2009, with the median rate of adoption increasing by about 1.5% per year from 2000 to 2007Reference Horowitz, Ebel and Ueda⁴⁷. Marriott and WanderReference Marriott and Wander⁴⁵ note that mean change in SOC in nine organic cropping systems they studied was 0.35 t C ha⁻¹ yr⁻¹, which is similar to that documented for NT systems (0.36–0.43 t C ha⁻¹ yr⁻¹) in various regions of the USReference Eve, Sperow, Paustian and Follett^48–Reference Johnson, Reicosky, Allmaras, Sauer, Venterea and Dell⁵⁰.

Differences in SOC were variable from LTARs that include both organic and NT systems, depending on the level of organic inputs. At the W.K. Kellogg Biological Station Long-Term Ecological Research (KBS LTER) site in Michigan, SOC in an organic system (10.2 Mg C ha⁻¹), to which no animal manure was added, was 22% less than in a NT (12.4 Mg C ha⁻¹) system after 10 yearsReference Robertson, Paul and Harwood¹⁶. Although not reported, carbon inputs, based on aboveground net primary productivity data and management information, appear to be similar in the NT and organic systems. Differences in soil C, then, appear to be due to differences in tillage. SOC was measured only to a depth of 7.5 cm in this study since differences between NT and tilled systems are generally seen at these surface depths.

At the Wisconsin Integrated Cropping Systems Trial (WICST) there was greater SOC at 0–5 cm in a NT corn–soybean (CS2; 26.5 g kg⁻¹) than an organic corn–soybean–wheat/red clover (CS3; 21.9 g kg⁻¹) rotation but no differences at 5–20 cm after 18 yearsReference Jokela, Posner, Hedtke, Balser and Read¹⁸. Carbon stocks in the WICST (0–20 cm), calculated from reported soil bulk density and C concentrations, were similar in the NT (56.0 Mg C ha⁻¹) and organic (57.6 Mg C ha⁻¹) systems. Even though the organic system (CS3) included cover crops and manure, C inputs were about 25% lower in the organic than the conventional NT system (CS2), due to lower crop residue yields in the organic system, suggesting that the form or placement of C inputs may have impacted soil C stocks.

At the Sustainable Agriculture Demonstration Project (SADP), which was recently concluded in Beltsville, Maryland, Teasdale et al.Reference Teasdale, Coffman and Mangum²⁰ showed that SOC concentration to a depth of 30 cm was greater in a legume (crimson clover) cover crop plus dairy manure-based reduced-tillage organic system than a NT system after 9 years (13.9 and 10.2 g kg⁻¹, respectively). Soil C concentration on a volume basis was not reported in this study. At another LTAR in Beltsville, Maryland, the Farming Systems Project (FSP), SOC to a depth of 1 m in corn–rye–soybean–wheat/legume rotations was 11% greater in a manure-based organic (Org3; 60.8 Mg C ha⁻¹) than in a NT (54.9 Mg C ha⁻¹) system after 11 yearsReference Cavigelli, Djurickovic, Mirsky, Maul and Spargo²². The NT system had not received manure for at least 14 years. Carbon inputs to the soil were greater in the organic than the NT systems in both Maryland studies, largely due to manure and/or compost additions. Results indicate that tilling sufficient organic materials, particularly manure, into soil may be a more effective means of increasing SOC than eliminating tillage.

Distribution of SOC with soil depth also differed substantially between the organic and the NT system at the FSP. While SOC in surface soils (0–5 cm depth) was greater in NT than in the organic system, SOC was substantially greater in Org3 than in NT at 5–10 and 10–25 cm depths (Fig. 1). Burying C inputs (poultry litter, ∼4.5 Mg ha⁻¹ every 3 years; and cover crop and crop residues) thus provided protection from repeated tillage in Org3. By contrast, SOC in the surface of NT systems is susceptible to loss following tillageReference Grandy, Robertson and Thelen⁵¹. Since the majority of farmers using NT do not use continuous NTReference Spargo, Alley, Follett and Wallace⁵², results from continuous NT research sites represent an upper limit to C sequestration levels likely achieved on-farm. Results from FSP also suggest that SOC in the organic system at KBS might be greater than indicated by only sampling to 7.5 cm depth. As Doran et al.Reference Doran, Sarrantonio, Liebig and Sparks⁵³ showed, soil samples should be taken to a minimum depth of 30 cm when comparing SOC in conventional and organic management systems.

Figure 1.

SOC with depth in 3-year NT and organic corn–soybean–wheat/legume crop rotations at the USDA-ARS Beltsville FSP 11 years after the initiation of the experiment.

SOM and soil fertility

SOM provides a number of supporting and regulating services. Among these benefits, increasing SOM increases soil fertility, reduces global warming potential (GWP) by sequestering atmospheric CO₂ in the soil, and, especially at the soil surface, increases water infiltration and helps stabilize soil to resist erosion.

A number of studies have shown that increasing SOM increases soil N fertility (N mineralization potential) in tilled organic systemsReference Clark, Horwath, Shennan and Scow^42, Reference Liebig and Doran^43, Reference Marriott and Wander^45, Reference Reganold, Palmer, Lockhart and Macgregor^54, Reference Drinkwater, Letourneau, Workneh, van Bruggen and Shennan⁵⁵. The relative impact of organic versus NT management on N fertility in organic systems seems to be related to SOM source and/or input levels. At the KBS LTER, where C inputs appear similar in a NT and an organic system, N mineralization potential was greater in the NT than the organic system after 14 years (http://lter.kbs.msu.edu/datatables/56). As with SOC, N mineralization potential seems driven primarily by differences in tillage when organic matter inputs are similar. At the WICST, although C inputs were greater in a NT than in an organic system, there was no difference in N mineralization potential between the two systemsReference Jokela, Posner, Hedtke, Balser and Read¹⁸. These results indicate that the quality rather than the quantity of N inputs may have impacted soil N fertility. At the FSP in Maryland, N mineralization potential was greater in the organic than the NT system by 34% after 14 yearsReference Spargo, Cavigelli, Mirsky, Maul and Meisinger²³. Greater soil N fertility resulted in 54% greater corn grain yield in the organic than the NT system in microplots to which no N source was added in year 15. At the SADP, also in Maryland, pre-side-dress soil nitrate, an in-season measure of plant N availability, was 42% greater and corn yield was 18% higher in the organic than the NT system during a uniformity trial following 9 years of experimental treatmentsReference Teasdale, Coffman and Mangum²⁰. While it is not possible to separate the impact of organic N input level versus source in these studies, it is likely that both factors played a role in augmenting soil N fertility. In any case, results from FSP indicate that N fertility in manure-based organic systems can be augmented relative to NT systems (non-manure based) in the long run even with relatively conservative rates of animal manure application (∼4.5 Mg ha⁻¹ poultry litter every 3 years).

SOM and GWP

GWP is the balance between the net exchange of the greenhouse gases CO₂, N₂O and CH₄ resulting from on-farm practices and the production and transport of inputs, and is generally driven by changes in SOC and emissions of N₂O in upland cropping systemsReference Robertson and Grace⁵⁶. GWP is expressed in units of CO₂ equivalents to account for the GWPs of CH₄ and N₂O being 25 and 298 times, respectively, that of CO₂⁵⁷. Rate of change in SOC was the primary factor driving differences in GWP among cropping systems at both the KBS LTER and FSP sitesReference Robertson, Paul and Harwood^16, Reference Cavigelli, Djurickovic, Mirsky, Maul and Spargo²². To our knowledge, these are the only two studies available comparing measured GWP between organic and NT systems; both studies also include CT systems.

At both KBS and FSP, GWP was lower in the organic than the CT system, primarily due to greater SOC in the organic than the CT systems. Other factors contributing to lower GWP in organic than CT included avoiding CO₂ emissions associated with N fertilizer production and transport (indirect energy use) and, at KBS, avoiding CO₂ emissions associated with dissolution of lime applied to soils.

At KBS, GWP was lower in the NT than the organic system, but the opposite was observed at FSP. At KBS, CO₂ emissions from N fertilizer production and transport and from lime dissolution were offset by increases in SOC in NT. GWP was about 2.8-fold lower for the NT than the organic systemReference Robertson, Paul and Harwood¹⁶. By contrast, at FSP, Org3 had greater SOC and lower CO₂ emissions from indirect energy use (energy used to produce and transport agricultural inputs) than NT, which offset approximately two-fold greater N₂O emissions in Org3 than NT. The resulting GWP was negative in Org3 and positive in NTReference Cavigelli, Djurickovic, Mirsky, Maul and Spargo²². An important caveat for the FSP study is that poultry litter was assumed to be produced on-farm (transportation distance of 1 km). While this is not an uncommon situation on the Eastern Shore of Maryland, where there is an important broiler chicken industry, manure transport in other locations can be substantial. The CO₂ emissions due to energy use were equal between NT and Org3 if poultry litter was transported 42 km or 114 km for wheat and corn production, respectivelyReference Cavigelli, Djurickovic, Mirsky, Maul and Spargo²².

Soil erosion

Soil erosion is sometimes asserted to be lower from organic than from conventional systems due to the erosion-protecting properties of additional SOM in organic systemsReference Pimentel, Hepperly, Hanson, Douds and Seidel^4, Reference Auerswald, Kainz and Fiener^58, Reference Papadopoulos, Bird, Whitmore and Mooney⁵⁹. However, as noted by Siegrist et al.Reference Siegrist, Schaub, Pfiffner and Mader⁶⁰ reduced soil erodibility in organic compared with conventional systems is not necessarily sufficient to protect against soil erosion during a heavy summer rainstorm.

There are very few direct measurements of soil erosion from organic versus conventional systems. A recent study from England showed lower interrill erosion following simulated rainfall from a silt soil managed organically versus conventionallyReference Kuhn, Armstrong, Ling, Connolly and Heckrath⁶¹. Phosphorus content of eroded soil, however, was much greater from the organic than the conventional system. These differences did not reflect soil test P, which was lower in the organically managed soil. One study that quantified the impact of organic compared with conventional farming on soil loss showed that soil depth after 37 years of farming was 21 cm greater on an organic farm than a neighboring conventional farm in the Palouse region of the USReference Reganold, Elliott and Unger⁶². These authors attributed lower soil erosion to greater use of cover crops in the organic system rather than to the soil-erosion controlling properties of additional SOM. Soil erosion is often estimated using mathematical models due to challenges associated with measuring soil erosion directly. When the Water Erosion Prediction Project (WEPP)Reference Flanagan and Nearing⁶³ model was applied to similar 3-year organic (Org3) and CT rotations at the FSP, predicted sediment loss was reduced by 33% in Org3 compared with CT (Fig. 2)Reference Green, Cavigelli, Dao and Flanagan²⁴. Losses of soil P, N and C in sediment runoff followed a similar pattern. Lower losses in Org3 than CT were due primarily to the presence of a winter legume cover crop following wheat harvest in Org3, a period during which there was no winter cover crop in the CT system. Thus, it seems that reduced soil erosion and nutrient runoff in organic compared with tilled conventional systems is due, at least in part, to greater use of cover crops in organic systems, and the role of greater SOM and associated soil properties is not clear.

Figure 2.

Sediment loss from CT, organic and NT cropping systems at the USDA-ARS FSP, Beltsville, Maryland, USA predicted using the WEPP model. All systems are corn–soybean–wheat/legume rotations; simulations are for a Mattapeake soil on 5% slope, 60 m slope length and 100-year timelineReference Flanagan and Nearing⁶³.

While we are not aware of any direct measurements of soil erosion in organic compared with NT systems—which are known to reduce soil erosion substantially compared with CT systemsReference Triplett and Dick¹⁵—it is very likely that soil erosion is lower in NT than organic systems due to differences in tillage. When the WEPP model was applied to 3-year rotations at FSP, soil erosion was reduced 80% in NT compared with Org3, with commensurate decreases in losses of soil P, N and C in sediment runoff (Fig. 2)Reference Green, Cavigelli, Dao and Flanagan²⁴. However, predicted soil erosion from a reduced-tillage organic system was similar to that from NT (3.7 versus 3.5 Mg ha⁻¹) at the SADP, based on simulations with the EPIC modelReference Watkins, Lu and Teasdale²¹, suggesting that decreased tillage could enhance erosion protection in organic systems.

Crop yield

Crop yield in organic systems has received considerable research attentionReference Connor^13, Reference Lotter^64, Reference Badgley, Moghtader, Quintero, Zakem, Chappell, Aviles-Vazquez, Samulon and Perfecto⁶⁵. A recent meta-analysis shows that organic grain yields, on average, were lower by 26% than those in conventional systems while organic oilseed yields were similar to those in conventional systems (though variability was high for oilseed data)Reference Seufert, Ramankutty and Foley⁶⁶. As noted by the authors, studies included in this meta-analysis were more rigorously selected than previous studies of this nature.

Lower yields in organic than conventional grain crops are usually related to challenges associated with timely weed control and/or nutrient supplyReference Smith, Menalled and Robertson^17, Reference Posner, Baldock and Hedtke^19, Reference Cavigelli, Teasdale and Conklin^25, Reference Teasdale and Cavigelli^26, Reference Coulter, Sheaffer, Wyse, Haar, Porter, Quiring and Klossner^32, Reference Porter, Huggins, Perillo, Quiring and Crookston^33, Reference Berry, Sylvester-Bradley, Philipps, Hatch, Cuttle, Rayns and Gosling⁶⁷. One study examined this annual variability in detail and found that corn and soybean yields in organic systems were 74% of those in conventional systems in about 1 of 3 years (over 21 site years), largely because of poor weed control in years with wet soils in the spring. By contrast, during the other 2 of 3 years weed control was effective and corn and soybean yields were essentially the same in organic and conventional systemsReference Posner, Baldock and Hedtke¹⁹. These results and those of Seufert et al.Reference Seufert, Ramankutty and Foley⁶⁶—showing that organic yields approach those of conventional systems when best management practices are used—indicate that despite a dearth of research on organic systems, organic management has the potential to produce crop yields similar to those in conventional systems. Additional research and development, including crop breeding, improved agronomic practices and improved engineering of weed control implements, will be needed to improve the consistency of organic system performance.

Some studies have reported greater grain yields in organic than tilled conventional systems during drought years and have attributed these differences to increased soil water-holding capacity in organic systemsReference Pimentel, Hepperly, Hanson, Douds and Seidel^4, Reference Lotter⁶⁴, another benefit of elevated SOM. Other studies, which tend to be from the southeast US, show that improved SOM and quality in organic systems does not necessarily result in greater yield in organic systems during dry yearsReference Cavigelli, Teasdale and Conklin²⁵ (Chris Reberg-Horton, personal communication). Soils in this region of the country are highly weathered, are more vulnerable to crop stress during droughty summer months, and have less tolerance to weed competition than, for example, Midwestern Mollisols. Variability of results highlights the site-specific nature of cropping systems performance and the need for site-specific research to improve organic systems.

Labor requirements and economic performance

Due largely to the multiple tractor passes required to control weeds in organic systems, organic farming has greater labor and management requirements than conventional methodsReference Hanson, Lichtenberg and Peters^29, Reference Mirsky, Ryan, Curran, Teasdale, Maul, Spargo, Moyer, Grantham, Weber and Way⁶⁸. Concerns about these labor requirements are frequently noted by organic grain farmers as a constraint to increasing organic grain acreageReference Ryan, Mortensen, Wilson and Hepperly⁶⁹. Critical shortage of organic feed corn throughout the US has required organic beef, dairy and poultry producers to purchase feed from distant locations. While the number of organic dairy farms in the US increased by 60% in 2008, the number of organic crop acres increased by only 8%Reference Benson⁷⁰. The US organic poultry industry experienced similar growthReference Dimitri and Oberholtzer⁷¹. In 2008, certified organic grain cropland was less than 1% of the total acreage while proportions dedicated to corn and soybean were only about 0.2%⁷².

The number of grain farmers adopting organic methods has been limited by at least four factors: (1) production challenges, including pest and fertility managementReference Cavigelli, Teasdale and Conklin²⁵; (2) perceived risks of returning to tillage-based systems by conventional grain farmers accustomed to the advantages of NT (Aaron Cooper, Farmer, personal communication); (3) high labor and fuel costs that prevent scaling up (Bill Mason and Ed Fry, Farmers, personal communication); and (4) lack of adequate information on sustainable management practices for organic systems among agricultural professionals, including those working for Cooperative Extension and the Natural Resources Conservation Service (NRCS)Reference Mainville, Farrell, Groover and Mundy⁷³.

Despite greater labor requirements and frequently lower grain yields, economic returns for organic systems in North America are generally greater than for CT and NT systems on a per hectare basis, due to substantial economic premiums received for organic grain cropsReference Cavigelli, Hima, Hanson, Teasdale, Conklin and Lu^27, Reference Mahoney, Olson, Porter, Huggins, Perillo and Crookston^34–Reference Delate, Duffy, Chase, Holste, Friedrich and Wantate^36, Reference Archer and Kludze^38, Reference Archer, Jaradat, Johnson, Weyers, Gesch, Forcella and Kludze^39, Reference Clark, Klonsky, Livingston and Temple^74–Reference Chavas, Posner and Hedtke⁷⁷. However, on a whole-farm basis, returns for organic farms may be lower than for conventional farms due to the smaller size of organic farms⁷⁸. Thus, labor-saving practices should help increase the number of acres planted to organic grains and improve the economic performance of organic farms.

In summary, results from a limited number of studies indicate that organic cropping systems that include legume cover crops and animal manures can result in greater SOC and soil fertility levels and lower GWP than NT systems, as long as manure transport distance is relatively short. In addition, economic returns on a per hectare basis are usually greater for organic than conventional systems. However, soil erosion is likely lower in NT than tilled organic systems and grain yields and whole-farm economic returns tend to be lower while labor requirements tend to be greater on organic than conventional farms.

Grain yield

Increasing the phenological diversity of crops in a rotation can increase yields of organic grain crops. At the FSP (Table 2) corn grain yield in a 6-year rotation (Org6) that includes summer annual (corn and soybean), winter annual (winter wheat) and herbaceous perennial (alfalfa) cash crops was, on average, 10% greater than in a 3-year rotation that includes only summer and winter annual cash crops (Org3) and 30% greater than in a 2-year rotation that includes only summer annual cash crops (Org2) (Table 3). These differences were the result of both increases in N availability and decreases in weed competition as crop rotation length and complexity increasedReference Cavigelli, Teasdale and Conklin^25, Reference Teasdale and Cavigelli^26, Reference Teasdale, Mangum, Radhakrishnan and Cavigelli²⁸. In Org2, opportunities to kill weeds occur at the same time each year since the two cash crops, corn and soybean, are planted at similar times. Thus, summer annual weeds (primarily Amaranthus spp., Chenopodium album, Daturum stramonium, Setaria spp., and Abutilon theophrasti) are favored in this system. When wheat is added to the rotation (Org3), the summer annual weeds either do not germinate or do not reach reproductive maturity as they are cut prior to setting seed when the wheat is harvested, and killed when soil is prepared for planting cover crops after wheat harvest. In Org6, a perennial forage crop, alfalfa, provides an additional level of phenological complexity that provides further weed control opportunities. Alfalfa is cut three to five times per year, a disturbance regime that tends to favor perennial and annual grasses with a prostrate growth habit rather than annual broadleaf weeds. Tillage prior to corn planting provides control of the grasses favored during the alfalfa phase of the rotation. Corn yield loss to weeds, as measured in adjacent weed-free and weedy plots, was reduced from 35% in Org2 to 14% in Org6 (Table 3), whereas, to provide context, yield loss to weeds in the 3-year conventional NT rotation was 7%Reference Teasdale and Cavigelli²⁶.

Table 3.

Impact of increasing crop phenological diversity on corn grain yields, weed pressure, economic risk, soil fertility, reliance on animal manure inputs, soil test phosphorus, soil erosion and soil N₂O emissions at the USDA-ARS Beltsville FSP.

¹ C = corn (maize; Zea mays L.); r = cereal rye (Secale cereale L.) cover crop; S = soybean (Glycine max L.); v = hairy vetch (Vicia villosa Roth.) cover crop; W = winter wheat (Triticum aestivum L.); F, perennial forage crop, which, from 1996 to 2000, was red clover (Trifolium pratense L.) and orchardgrass (Dactylis glomerata L.); since 2000, has been alfalfa (Medicago sativa L.).

² 75% lower confidence limit of net returns (a smaller lower limit represents a greater risk), as reportedReference Dimitri and Oberholtzer⁷¹.

³ Mehlich 3 extractable (M.A. Cavigelli, unpublished data).

⁴ Calculated using the RUSLE2 for a soil with 2–5% slope and an erodibility factor (K) of 0.28. Results were similar with other settings (T. Pilkowski and M.A. Cavigelli, unpublished data).

⁵ Means of measurements made in 2008 and 2010 (Org3) or 2010 only (Org2 and Org6), fromReference Singer, Franzluebbers, Karlen, Wedin and Fales⁸⁰. Research supported by GRACEnet, http://www.ars.usda.gov/research/programs/programs.htm?np_code=204&docid=17271.

Organic grain yields also increased with increasing phenological diversity of crops in a rotation at the two Variable Input Cropping Management Systems studies (VICMS1 and VICMS2) in Lamberton, Minnesota (Table 2). Corn grain yield was 26% (VICMS1) and 50% (VICMS2) greater in a 4-year oat/alfalfa–alfalfa–corn–soybean than a 2-year corn–soybean crop rotationReference Coulter, Sheaffer, Wyse, Haar, Porter, Quiring and Klossner^32, Reference Porter, Huggins, Perillo, Quiring and Crookston³³. Weed pressure was lower at the VICMS2 site, which had a history of pesticide applications, than at the VICMS1 site, which did not have a history of pesticide applications. At VICMS1 there was no impact of rotation length on organic soybean yield but at VICMS2 soybean yield was 42% greater in the 4-year than the 2-year rotation. In Germany, wheat yields in organic systems were found to be 31% greater when a perennial forage was part of the rotation than when only cash crops were part of the rotationReference Schmidt, Schulz, Leithold, Raupp, Pekrun, Oltmanns and Kopke⁷⁹.

In other studies, phenological diversity of crops in an organic rotation did not impact crop yields. At the Farming Systems Trial (FST) in Kutztown, PA, there was no difference in corn or soybean yields in a 3-year corn–small grain–soybean–small grain–legume rotation compared with a 5-year corn–rye–soybean–silage corn–wheat/red clover–alfalfa rotation over a 16 year time periodReference Hepperley, Douds, Seidel, Raupp, Pekrun, Oltmanns and Kopke³⁰. At the WICST there was no difference in corn yield between a 3-year corn–soybean–wheat/red clover (CS3) and a 5-year corn–soybean–oat/pea–alfalfa–alfalfa rotation (CS5)Reference Posner, Baldock and Hedtke¹⁹. Differences between rotations in these two studies, however, are more subtle than in studies comparing rotations with and without perennial forages (e.g., FSP and VICMS). In Greenfield, Iowa and Morris, Minnesota (Table 2), there were also no impacts of crop phenological diversity during the first 3 or 4 years of experiments comparing 2- or 3- and 4-year rotationsReference Delate, Duffy, Chase, Holste, Friedrich and Wantate^36, Reference Delate and Cambardella^37, Reference Archer, Jaradat, Johnson, Weyers, Gesch, Forcella and Kludze³⁹. These results may reflect that these studies report data for only the first 3 or 4 years following plot establishment. Weed populations in organic fields that had previously been managed conventionally are usually low but can increase with time in organic management. In the Iowa study, there are also only subtle differences between the 3- and 4-year rotations (Table 2). While rotations that include perennial forages can increase grain yields compared with simple corn–soybean rotations, there is a need to better understand when and how phenological diversity of crops in organic rotations impacts crop yields.

Economic performance

Increasing crop phenological diversity provides some economic benefits. Crops with different phenology tend to respond differently to weather fluctuations such that the economic performance of a more diverse rotation is better buffered against variable weather. At the FSP, net returns for the 6-year rotation that includes a perennial forage, Org6, were substantially greater than the mean for Org2 and Org3 when no organic price premiums were included. Differences were due to lower production costs and greater returns in the longer rotationReference Cavigelli, Hima, Hanson, Teasdale, Conklin and Lu²⁷. When price premiums for corn, soybeans and wheat were included in the analysis, net returns for the three organic systems were similar and substantially higher than without premiums. However, economic risk, measured using the 75% lower confidence limit of net returns, was 7.5 and 3.9 times greater for Org2 and Org3, respectively, than for Org6 (Table 3). At the VICMS project, Mahoney et al.Reference Mahoney, Olson, Porter, Huggins, Perillo and Crookston³⁴ also found lower risk (and greater return) for a 4-year corn–soybean–oats/alfalfa–alfalfa rotation than a 2-year corn–soybean rotation. On the other hand, researchers in Iowa and Morris, Minnesota found no difference in risk between 2- or 3- and 4-year crop rotations, respectivelyReference Delate, Duffy, Chase, Holste, Friedrich and Wantate^36, Reference Smith, Clapperton and Blackshaw⁷⁶. These results, again, might reflect that these studies report on the first three transition years of these experiments.

Nutrient management

Increasing crop phenological diversity can benefit soil nutrient management. At the FSP, N mineralization potential, particulate organic matter N, and SOC were similar among the three organic systems and all were greater than in CT and conventional NT systemsReference Spargo, Cavigelli, Mirsky, Maul and Meisinger²³. Interestingly, the longest of the three organic rotations, Org6 relies on fewer external inputs of poultry litter than the two shorter rotations (Org2 and Org3; Table 3). During a 6-year time period, typical poultry litter application rates were 13.4, 17.9 and 9.0 Mg ha⁻¹ in Org2, Org3 and Org6, respectively. Since P removal in harvested crops was greater in Org6 than Org2 and Org3, soil test P was 21% lower in Org6 than in Org2 and Org3 after 16 years (Table 3). Thus, the possibility of overloading soils with phosphorus, an important concern in many watersheds, especially when animal manures are applied, was reduced considerably with Org6 compared with the shorter rotations.

Soil erosion

Increasing crop phenological diversity also substantially decreased predicted soil erosion among organic systems at the FSP. When the Revised Universal Soil Loss Equation, Version 2 (RUSLE2) was applied to the three FSP organic systems, predicted soil loss by erosion was reduced by 40 and 62% in Org3 and Org6, respectively, compared with Org2 (Table 3). These results are consistent with the general finding that small grain crops and perennial forages can reduce soil erosion compared with row crops such as corn and soybeanReference Singer, Franzluebbers, Karlen, Wedin and Fales⁸⁰.

Soil N₂O emissions

Preliminary results from the FSP indicate that annual soil N₂O emissions were reduced by about 50% in the 6-year rotation that includes a perennial forage, Org6, compared with the shorter organic rotations, Org2 and Org3 (Table 3). This represents a N₂O mitigation potential equal to the best mitigation options in agriculture in the Eastern and Central USReference Cavigelli, Parkin, Liebig, Franzluebbers and Follett⁸¹. This reduction was due to a decrease in the proportion of high N demanding crops in the Org6 rotation compared with the two shorter rotations. Although GWP was not calculated for Org6, reducing N₂O emissions using a more diverse crop rotation that includes proportionally fewer high N-demanding crops should help decrease GWP considerably given that N₂O was a dominant source of GWP at both the KBS LTER and at FSP.

While integrating perennial forages into annual grain cropping systems is an effective way to reduce tillage and improve a number of ecosystem services (Table 3), many producers are reluctant or not able to produce perennial forages due to limitations in equipment, expertise and/or markets. In many areas, development of confined animal feeding operations has concentrated animal production such that large areas have historically low animal populations and forage demand is lowReference Russelle, Entz and Franzluebbers^82, Reference Watson, Walker and Stockdale⁸³. Some organic farmers, like their conventional neighbors, are therefore limited to producing primarily grain crops. Thus, there is a need to improve provisioning, regulating and supporting ecosystem services provided by organic rotations that do not include perennial foragesReference Watson, Walker and Stockdale⁸³.

The following sections address research designed to improve ecosystem services provided by organic cropping systems, including improving nutrient management and reducing tillage. Since these are relatively new areas of research, there are fewer data available to assess the viability of these approaches. However, expanding organic grain acreage in the US to meet increasing demand for organic meat and milk products will likely require that we improve existing organic production systems to address farmer concerns with labor (largely tillage) requirements and nutrient management concernsReference Watson, Bengtsson, Ebbesvik, Loes, Myrbeck, Salomon, Schroder and Stockdale⁸⁴ of traditional organic grain cropping systems. These studies highlight promising approaches to improve ecosystem services.