Abstract
AbstractAgricultural crop rotations have been shown to increase soil carbon (C), nitrogen (N), and microbial biomass. The mechanisms behind these increases remain unclear, but may be linked to the diversity of crop residue inputs to soil organic matter (SOM). We used a residue mixture incubation to examine how variation in long‐term diversity of plant communities in agroecosystems influences decomposition of residue mixtures, thus providing a comparison of the effects of plant diversification on decomposition in the long term (via crop rotation) and short term (via residue mixtures). Three crop residue mixtures, ranging in diversity from two to four species, were incubated for 360 d with soils from five crop rotations, ranging from monoculture corn (mC) to a complex five‐crop rotation. In response, we measured fundamental soil pools and processes underlying C and N cycling. These included soil respiration, inorganic N, microbial biomass, and extracellular enzymes. We hypothesized that soils with more diverse cropping histories would show greater synergistic mixture effects than mC. For most variables (except extracellular enzymes), crop rotation history, or the long‐term history of plant diversity in the field, had a stronger effect on soil processes than mixture composition. In contrast to our hypothesis, the mC soil had nearly three and seven times greater synergistic mixture effects for respiration and microbial biomass N, respectively, compared with soils from crop rotations. This was due to the low response of the mC soils to poor quality residues (corn and wheat), likely resulting from a lack of available C and nutrients to cometabolize these residues. These results indicate that diversifying crop rotations in agricultural systems alter the decomposition dynamics of new residue inputs, which may be linked to the benefits of increasing crop rotation diversity on soil nutrient cycling, SOM dynamics, and yields.
Highlights
It is critical that we understand the linkages between plant biodiversity and soil processes, especially in ecosystems where aboveground biodiversity is already low, such as in agroecosystems
We found that crop rotation legacy exerted more control over the mixture effect for CO2 production, inorganic N, and microbial biomass than the chemical characteristics or diversity of residue mixtures (Table 3), but that residue mixture influenced enzyme activities (EEA)
In a previous paper from the same crop rotation experiment, we reported that increasing cropping system biodiversity increased soil microbial biomass, increased potentially mineralizable C and N, and increased the activities of extracellular enzymes that are responsible for labile vs. recalcitrant C decomposition (McDaniel et al 2014a)
Summary
It is critical that we understand the linkages between plant biodiversity and soil processes, especially in ecosystems where aboveground biodiversity is already low, such as in agroecosystems. Even though agroecosystems are notoriously low in plant diversity, farmers have been increasing aboveground biodiversity through crop rotations for millennia. It is unclear, whether this form of increasing plant diversity through time alters plant residue decomposition. Increased plant diversity could promote positive soil feedbacks on residue decomposition and soil organic matter (SOM) stabilization, as recently shown in grasslands (Lange et al 2015), and may contribute to C and N accumulation in soils with rotated crops (McDaniel et al 2014b). A nonadditive effect occurs when an observed response to mixtures is different from that predicted by averaging the response of the individual litters The latter is further classified as either nonadditive synergistic (NAS, positive) or nonadditive antagonistic (NAA, negative), but most observed mixture effects have been NAS (Gartner and Cardon 2004)
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