Long-term biogeochemical cycling in agroecosystems inferred from 13C, 14C and 15N

We investigated C and N cycling in long-term agroecological experiments initiated over 50 years ago at a cool, semi-arid site on the North American Great Plains. We used isotopes at natural abundance to trace C and N exchange between soils and plants in contrasting cropping systems. Both 13C and 14C indicated that the soil organic matter was isotopically distinct from current plant inputs, suggesting that recently added plant C was cycling independently of much of the soil C pool. For tracing recent C flows, bomb-14C was more sensitive than 13C, and increased more in high – than in low – yielding systems. Analysis of 15N in plant tissues, as an index of 15N in actively cycling soil N, suggested that biological and industrial N fixation both tended to decrease plant 15N, whereas livestock manure addition increased 15N abundance. Collectively, the data suggest that soil organic matter is kinetically heterogeneous, so that a majority of soil C and N inputs and outputs exchange with only the small pool of soil organic matter that is actively cycling. Consequently, recently photosynthesized C and deposited N may not readily enter the old, stable fractions of soil organic matter. Practices to retain CO2 from the atmosphere and prevent leakage of reactive N to non-agricultural systems should therefore focus on management of this active pool.