Feedbacks between nitrogen fixation and soil organic matter increase ecosystem functions in diversified agroecosystems

Abstract

Nitrogen (N) losses from intensified agriculture are a major cause of global change, due to nitrate (NO 3 À ) export and the eutrophication of aquatic systems as well as emissions of nitrous oxide (N 2 O) into the atmosphere. Diversified agroecosystems with legume cover crops couple N and carbon (C) inputs to soil and reduce N pollution, but there is a need to identify controls on legume N 2 fixation across ecosystems with variable soil conditions. Here, I tested the hypothesis that N mineralization from turnover of soil organic matter (SOM) regulates legume N 2 fixation across 10 farms that spanned a gradient of SOM levels. I separated soil samples into two SOM fractions, based on size and density, which are indicators of soil nutrient cycling and N availability (free particulate organic matter and intra-aggregate particulate organic matter [POM]). This study indicates downregulation of legume N 2 fixation in diversified agroecosystems with increasing N availability in intra-aggregate POM and increasing N mineralization. Intercropping the legume with a grass weakened the relationship between N in POM and N 2 fixation due to N assimilation by the grass. Further, mean rates of N and C mineralization across sites increased with two seasons of a legume-grass cover crop mixture, which could enhance this stabilizing feedback between soil N availability and N 2 fixation over time. These results suggest a potential mechanism for the diversity-ecosystem-function relationships measured in long-term studies of agroecosystems, in which regular use of legume cover crops increases total soil organic C and N and reduces negative environmental impacts of crop production.

Soil fertility status controls the decomposition of litter mixture residues

“…Soil fertility indicators, such as POM and related labile C and N pools, respond relatively quickly to changes in management and are considered strong indicators of both soil nutrient cycling and C stabilization (Marriott and Wander 2006 a , Hurisso et al 2016, Blesh 2019). Accumulation of these organic matter pools, which include the microbial biomass itself, affect and are affected by the microbial community.…”

“…To test our hypotheses, we designed an aerobic incubation experiment using soil and litter collected from two farm fields in southeastern Michigan with contrasting levels of soil fertility, likely due to their different long‐term management histories (Appendix : Table ). Both fields had the same short‐term (i.e., two‐year) history, because they were involved in a companion experiment with the same organic management regime, which included two winters with a hairy vetch–cereal rye cover crop mixture when we began the current study (Blesh 2019). However, the fields had different long‐term legacies.…”

“…We also analyzed fractions of SOM that are important for microbial decomposition and nutrient cycling, and which tend to reflect different management histories (Drinkwater et al 1996, Wander 2004). Light fraction particulate organic matter (POM; also called free POM), and intra‐aggregate POM (i.e., physically protected, or occluded POM), was separated from triplicate 40 g subsamples of air‐dried soil using a size and density fractionation method (Marriott and Wander 2006 b , Blesh 2019). We retained material larger than 53 μm, and the C and N of free POM and protected POM were measured on an ECS 4010 CHNSO Analyzer (Costech Analytical Technologies; Valencia, California, USA).…”

“…For the incubation, we used dried and ground vetch and rye litter from the two fields, which had been collected from monoculture plots of each species in the first year of the experiment in May 2016. Each replicate block per field had included three microplots of 1 m 2 with sole vetch, sole rye, and vetch‐rye mix treatments (Blesh 2019). The biomass was sampled to ground level from a 0.25 m 2 quadrat in each plot in each field, avoiding soil, immediately before cover crop incorporation.…”

Ecological and Nutritional Functions of Agroecosystems as Indicators of Smallholder Resilience

“…These N and C inputs add to pools of bioavailable soil nutrients, as cover crops are generally not harvested but instead are incorporated into the soil at the end of the season as "green manures." This agroecological practice has therefore been shown to increase internal nutrient cycling and nutrient availability to primary crops, with potential to increase productivity over time (Wander et al, 1994;Blesh, 2019). More broadly, the addition of legume cover crops to crop rotations introduces additional plant traits that influence ecosystem functions, contributing to both agricultural biodiversity (agrobiodiversity) and crop functional diversity (Wood et al, 2015).…”