Cropping biodiversity effects on soil organic matter dynamics and microbial function

Background/Question/Methods

Crop rotations, which increase plant diversity through time, are considered key to sustainable agroecosystem management.  However, the exact mechanisms by which rotational diversity increases soil fertility and plant productivity remain unknown.  We used a combination of a dual-labeled (¹³C and ¹⁵N) wheat litter experiment and laboratory incubations to elucidate the effects of crop rotational diversity on soil organic matter (SOM) stabilization and microbial functioning.  The experiment was conducted at the Crop Biodiversity Gradient, which is part of the W.K. Kellogg Biological Station LTER.  This experiment includes replicated (n = 4) treatments ranging from 0 to 10 total plant species.  It was predicted that the most diverse cropping rotations would sequester more C and N from labeled litter, have greater SOM complexity, and possess microbial communities that utilize a more diverse suite of substrates. 

Results/Conclusions

Preliminary results from the dual-labeled litter experiment suggest that the proportion of CO₂-C from the wheat litter, relative to the native mineral soil C, was approximately 20% lower in the spring fallow treatment (highest diversity) than compared to corn-soy and corn-soy-wheat plus cover crop rotations.  Bulk mineral soil ¹³C and ¹⁵N concentrations suggest treatment differences in how new litter is incorporated into mineral SOM.  Microbial biomass, basal respiration, extracellular enzyme activities, and soil C and N concentrations were greatest in the most diverse rotation treatments (corn-soy-wheat) with cover crops.  An incubation experiment with single and mixed crop litter additions to soils from the Crop Biodiversity Gradient showed that both litter (p < 0.001) and soil (p < 0.001) had significant effects on respiration.  The cumulative respiration was driven mostly by litter and soil C:N ratios, which explained 77% of the variation in respiration rates.  We also used community-level physiological profiles (using 31 substrates) to examine whether the diversity of SOM inputs derived from crop rotations influences soil microbial substrate usage.  Overall, our results indicate that increasing aboveground biodiversity through time alters soil microbial communities and belowground biogeochemistry.