Cover Crops Do Not Give Rise to Positive Net Ecosystem C Balance.

The potential of an agroecosystem to increase soil C storage is dependent on a positive net ecosystem C balance (NECB), considered as the total quantity of biomass C input relative to C lost via harvest and soil respiration. Increasing soil C storage improves agroecosystems by promoting plant productivity and reducing erosion and nutrient loss. We quantified potential C storage as NECB in no-till corn (Zea mays L.) under grain or stover harvest management (whole-plot treatment) with annual winter rye (Secale cereale L.) or perennial Kentucky bluegrass (Poa pratensis L.) cover crops (sub-plot treatment) for 2015 and 2016. We found more rye biomass in stover harvest systems, particularly in 2016 (158 ± 49 g C m^-2 compared to 21.3 ± 5.3 g C m^-2 in grain systems). Bluegrass was equally productive under grain or stover harvest (75 ± 18 g C m^-2). Corn yield was not affected by cover crop, but stover harvest removed more C (916 ± 43 g C m^-2) compared to grain (505 ± 14 g C m^-2). Neither cover crop provided enough biomass to balance corn harvest, and the NECB was negative or equivalent to 0 in all systems. In addition, an 18-month litterbag study found that rate of plant litter loss, extracellular enzyme activity, and microfaunal abundance were similar among all treatments, suggesting that decomposition processes were not affected by cover crops. Although cover crops are widely believed to promote soil health and increase soil C storage, our results suggest that cover crops promote only minor shifts in C cycling, and that greater biomass growth is necessary for positive NECB. Combining mechanistic understanding of decomposition with field trials of cover crop agronomy should improve predictions of agroecosystem C storage and recommendations to land managers.