Despite increasing residue input in annual crop production systems, N fertilization does not increase soil organic carbon (SOC) over time because N fertilization also increases organic carbon (OC) decay. This study also shows that belowground OC inputs contribute to soil carbon sequestration more than aboveground OC inputs to the soil.
When all phases of the crop rotations were evaluated over the long term, OC decay rates increased concomitantly with OC input rates in several systems. Increases in decay rates with N fertilization apparently offset gains in carbon inputs to the soil in such a way that soil C sequestration was virtually nil in 78% of the systems studied, despite up to 48 years of N additions [Russell 2009: 1102].
Across all systems, SOC storage was significantly correlated with the quantity of belowground OM [organic matter] inputs (P < 0.01, both sites). In contrast, SOC was not correlated with the quantity of aboveground inputs (P = 0.45, Nashua; P = 0.55, Kanawha) [Russell 2009: 1111].
This study highlights the importance of incorporating both production and decomposition processes, as well as the location (above- or below-ground) of detrital inputs into models of N-fertilization effects on soil C dynamics in agroecosystems. These results are highly relevant for evaluating the potential of N fertilization to mitigate the effects of removal of organic-matter ‘‘residue’’ from the system for bioenergy production. Our data suggest that the stimulation of OC decomposition by the addition of fertilizer N would likely counteract the positive effects of N fertilization on inputs of OC to the soil, at least for annual crops. Given the current quantity of N that is applied over such a large area, management strategies that can maintain high yields and also reduce N-fertilizer use would also have beneficial environmental consequences. Our study indicates that selection of crops for higher belowground NPP [net primary production], in rotation with crops that fix N, could maximize both yields and soil C sequestration without excessive N-fertilizer additions [Russell 2009: 1111].
Russell, Ann E. et al, 2009, Nitrogen fertilizer effects on soil carbon balances in Midwestern U.S. agricultural systems, Ecological Applications 19(5), 1102-1113: https://esajournals.onlinelibrary.wiley.com/doi/10.1890/07-1919.1.