Net mineralization of soil N and 15N-ryegrass residues in differently textured soils of similar mineralogical composition

The net mineralization of soil nitrogen (N) and of 15N-labelled ryegrass residues was studied in twelve differently textured soils of similar mineralogical composition and cropping history. Six soils with 11, 16, 21, 31, 37 and 45% clay (termed NA1 to NA6) were from an arable field with a naturally occurring texture gradient. Three clay amended soils (CL2, CL4, CL6) and three silt amended soils (SI2, SI4, SI6) were prepared by spiking portions of the NA1 soil with clay and silt sized organomineral complexes isolated from the NA2 soil. 15N-labelled ryegrass and soil (<8 mm) were mixed and adjusted to -10 kPa before being incubated at 20 degrees C. Sufficient replicates were prepared to allow for ten destructive samplings during the 31 weeks of incubation. Following incubation, samples were analysed for 15N-labelled and unlabelled 1 M KCl extractable mineral N (NH4(+) + NO3(-)), total N and microbial biomass N using chloroform fumigation-extraction methodology. When described by first order kinetics, the pool of potentially mineralizable native soil N (N0) was similar in the three SI soils. N0 was not significantly affected by clay content in the NA soils. N0 decreased with increasing clay content in the CL soils and the proportion of N held in the N0 pool was smaller than in the NA soils. At the end of the incubation, 28-36% of the 15N applied with the ryegrass had become mineralized. The influence of clay differed for the three sets of soils. The NA soils showed a decreasing and the CL soils an increasing 15N-mineralization with increasing clay content. However, the effect of clay in the NA and CL soils was relatively small. 15N-turnover in the SI soils showed no response to soil texture. Between 12 and 16% of the 15N initially added with ryegrass was rapidly incorporated into the microbial biomass in the NA and CL soils. Less 15N (8-10%) was found in the biomass of the SI soils. 15N in the microbial biomass declined during incubation. At the end of the incubation, about 5% of the added 15N resided in microbial biomass regardless of soil characteristics. The differently textured NA soils were comparable in mineralogical composition, cropping history and moisture status during incubation. Under such conditions the textural composition of the soil appears to play a minor role for N turnover, suggesting that the capacity of soils to stabilize organically bound N in clay organomineral complexes is less important to short-term N dynamics.