Effects of species and functional diversity of crop rotations on soil organic matter: A meta-analysis
Diversified crop rotations have been linked to increased soil organic matter (SOM) at individual field sites, but a global synthesis of this relationship is lacking. We asked, what attributes of diversified crop rotations enhance total and labile pools of soil carbon (C) and nitrogen (N)? We sought to explore whether crop species diversity per se influences SOM or if the diversity of functional groups of crops, such as the addition of crop species other than annual grain crops (i.e., hay, pasture, or cover crops), would drive the accumulation of SOM. We compiled data from cropping system sites that tested at least two different levels of crop species diversity (e.g., 1 and 2 species crop rotations, or 2 and 4 species crop rotations). Explanatory variables included number of species in crop rotation, presence or absence of a hay, pasture, or cover crop, tillage, inorganic N fertilizer, and organic amendments. We also included rates of N inputs and grain and hay exports. Response variables included concentrations of soil organic C and total N, microbial biomass C and N, and potentially mineralizable N.
Preliminary data show that 70% of rotations included only annual grain crops and that rotations most commonly included two crop species (39% 2 species, 35% 1 species, 9% 3 species, 9% 4 species, 5% 5 species, 3% 6 species). Increasing species diversity was strongly linked to including a crop that served as a hay, pasture or cover crop: whereas only 16% of 2 species rotations included a hay, pasture, or cover crop, 87% of 4 species rotations and 100% of 6 species rotations did. Future work will include analysis of the relationship between crop rotation diversity (both species diversity and functional diversity) and C and N retention in soil organic matter. We hope to produce a global dataset that describes the ability of crop rotations to enhance soil organic C and N retention and to what extent these relationships hold across soil types and climatic regions.