Variation of stabilised, microbial and biologically active carbon and nitrogen in soil under contrasting land use and agricultural management practices

Land use and agricultural practices modify both the amounts and properties of C and N in soil organic matter. In order to evaluate land use and management-dependent modifications of stable and labile C and N soil pools, (i) organic C and total N content, (ii) microbial (Cmic) and N (Nmic) content and (iii) C and N mineralisation rates, termed biologically active C and N, were estimated in arable, grassland and forest soils from northern and southern Germany. The C/N-ratios were calculated for the three levels (i)–(iii) and linked to the eco-physiological quotients of biotic-fixed C and N (Cmic/Corg, Nmic/Nt) and biomass-specific C and N mineralisation rate (qCO2, qNmin). Correlations could mainly be determined between organic C, total N, Cmic, Nmic and C mineralisation for the broader data set of the land use systems. Generally, the mineralisation activity rate at 22 °C was highly variable and ranged between 0.11 and 17.67 μg CO2–C g−1 soil h−1 and −0.12 and 3.81 μg (δNH4+ + δNO3−)–N g−1 soil h−1. Negative N data may be derived from both N immobilisation and N volatilisation during the experiments. The ratio between C and N mineralisation rate differed significantly between the soils ranging from 5 to 37, and was not correlated to the soil C/N ratio and Cmic/Nmic ratio. The C/N ratio in the ‘biologically active’ pool was significantly smaller in soils under conventional farming than those under organic farming systems. In a beech forest, it increased from the L, Of to the Ah horizon. The biologically active C and N pools refer to the current microbial eco-physiology and are related to the need for being C and N use efficient as indicated by metabolic qCO2 and qNmin quotients.