Cropping biodiversity effects on decomposition of dual-labeled (13C and 15N) wheat residue

Background/Question/Methods

Crop rotations, which increase plant biodiversity through time, are considered key to sustainable agroecosystem management.  However, the exact mechanisms by which rotational diversity increases soil organic matter, fertility, and plant productivity remain unknown.  We used a dual-labeled (¹³C and ¹⁵N) wheat residue experiment to elucidate the effects of crop rotational diversity on soil processing of new plant inputs into stabilized soil organic matter (SOM).  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 7 total crop species.  Dual-labeled wheat was decomposed in mesocosms within three crop diversity treatments: corn-soy (C-S), corn-soy-wheat-red clover + rye cover (C-S-W-2cov), and spring fallow (SF, 5-7 native species).   It was predicted that the more diverse cropping rotations would lose less and sequester more C and N from labeled residue.

Results/Conclusions

Gas flux results showed that the overall flux of CO₂-C was not significantly different among crop rotations, however, the proportion of the CO₂-C originating from the newly added wheat residue was affected by crop rotational diversity (P < 0.001).  Specifically, the SF treatment had 13% and 15% less CO₂-C from the labeled wheat than the C-S and C-S-W-2cov treatments, respectively.  Preliminary data from bulk mineral soil ¹³C and ¹⁵N concentrations also suggested treatment differences in how the new wheat residue is incorporated into mineral SOM.  Soil ¹³C mirrored CO₂-¹³C data in that lowest effluxes (i.e. losses) were matched with greatest wheat ¹³C and ¹⁵N incorporated into SOM.  The ¹³C recovered, as % of applied, in bulk soils was 20, 7, and 1% in SF, C-S-W-2cov, and C-S respectively.  The ¹⁵N recovered, as % of applied, in bulk soils was 24, 10, and 2% in SF, C-S-W-2cov, and C-S respectively.  Proportion of the wheat C and N in microbial biomass pools, aggregate fractions, and leached through the soil profile were also measured.  Taken together, these results indicate that increasing biodiversity increases the proportion of C and N from wheat residue that is stabilized into SOM, and thus decreasing the proportion lost via gas flux or leaching.   The large differences among crop rotations in processing new wheat residue is indicative of a strong soil biological mechanism, primarily driven by decomposers, whereby higher-diversity rotations can sequester C and enhance soil fertility.