Impact of fertilizer type and rate on carbon and nitrogen pools in a sandy Cambisol

Type and rate of fertilizers influence the level of soil organic carbon (C-org) and total nitrogen (N-t) markedly, but the effect on partitioning of C and N into different pools is open to question. Objectives were to investigate the impact of fertilizer type and rate on labile, intermediate and passive C and N pools in a sandy Cambisol at Darmstadt, Germany, after 27 years of different fertilization treatments. The six treatments were: straw incorporation plus application of mineral fertilizer (MSI) and application of farmyard manure (FYM) each at high (140-150 kg N ha(-1) year(-1)), medium (100 kg N ha(-1) year(-1)) and low (50-60 kg N ha(-1) year(-1)) rates. Soil microbial biomass C (C-mic) and N (N-mic) and C and net N mineralization (266 days incubation at 10A degrees C and 50% waterfilled pore space) were determined. Soils (0-25 cm) of MSI treatments had significantly (p a parts per thousand currency signaEuro parts per thousand 0.05) lower C-mic stocks (308-361 kg ha(-1)) than soils of FYM treatments (404-520 kg ha(-1)). Differences in N-mic stocks were less pronounced. After 266 days, mineralized C (1130-1820 kg ha(-1)) and N (90-125 kg ha(-1)) had significantly increased with fertilizer rate. The application of an exponential two-pool model showed that very labile pools (turnover times: 17 and 9 days for C and N, respectively) were small (1.3-1.8% of C-org and 0.5-1.0% of N-t) and not influenced by type or rate of fertilizer. Stocks of the modeled labile C and N pools (turnover times: 462 and 153 days for C and N, respectively) were not influenced by the type of fertilizer but depended significantly on the application rate and ranged from 7 to 13% of C-org and from 4 to 5% of N-t. In contrast, the size of the calculated intermediate C pool was greater for the FYM treatments, and depended significantly on the interaction of fertilizer type and rate. The intermediate N pool was unaffected by fertilizer type or rate. Passive C and N pools, as experimentally revealed by oxidation with disodium peroxodisulfate (Na2S2O8), were independent of the treatments. Overall, labile and intermediate pools were affected differently by the fertilizer type and the application rate.