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

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.