Effect of fertilization on respiration from different sources in a sandy soil of an agricultural long-term experiment

Annual changes in stocks of soil organic carbon may be detected by measurement of heterotrophic respiration, but field studies of heterotrophic respiration in long-term fertilization experiments on sandy soils are scarce. Our objectives were to: (1) investigate the influence of fertilizer type on mineralization of soil organic carbon and crop residue, and (2) show how fertilization treatments affect the annual C balance (net ecosystem carbon balance, NECB; negative values indicate a CO2-source) in the sandy soil of the Darmstadt experiment. Treatments were long-term mineral fertilization with cereal straw incorporation (MSI) and application of rotted farmyard manure (FYM), both treatments receiving 14 g N m(-2) year(-1). This study used delta C-13 natural abundance after introduction of a C-4 crop to distinguish between different sources of respiration. Mineralization derived from C-3 sources was similar for MSI and FYM treatments (similar to 270 g C m(-2) year(-1)). The rate of residue mineralization in MSI treatments was higher, resulting in a mineralization of 49 and 37% of initial residue C in the soil of MSI and FYM treatments, respectively. The NECB (g C m(-2) year(-1)) indicated the MSI treatment (approximately - 190) as a stronger source compared with the FYM treatment (similar to-30).