The influence of the soil matrix on nitrogen mineralisation and nitrification. IV. Texture

Small undisturbed soil volumes (c. 1.7 cm(3)) were collected from the surfa ce of a small field plot. Soil volumes were treated with clover-derived sub strate, dried and rewetted, or retained continuously moist from the field. These soil volumes were then incubated for 20 days at a matric water potent ial of either -10 or -30 kPa. At the end of the incubation the soil was ana lysed for volumetric water content (theta v), NO3--N, NH4+-N, total N (%N), and percentages of sand, silt, and clay. The texture terms were included i n linear regression models, together with %N and theta v as predictors of N mineralisation and nitrification. Clay and sand were often observed to have a significant influence on N mine ralisation and nitrification, but silt rarely appeared to influence these p rocesses. In soils retained continuously moist, %clay had a negative relati onship with N mineralisation and nitrification, but this relationship was p ositive in soils that had been dried and rewetted. The results suggest that during periods of relatively high moisture content, soils that are higher in clay are able to protect organic N more effectively from microbial attac k. However, on drying and rewetting, the protective mechanisms of clay are undermined, the relatively large protected reservoirs of organic N in high clay soils become more vulnerable to microbial attack, and these soils ther efore experience a greater flush of N mineralisation than soils with lower clay levels. The negative influence of clay in the continuously moist soils was not as clearly observed in the soils incubated at -10 kPa as in soils incubated at -30 kPa, suggesting that the decomposition of organic N reside nt in larger pores (10-30 mu m neck diameter) may not be as strongly regula ted by clay as that resident in smaller pores. When soils were treated with clover-derived substrate, clay had a positive relationship with N minerali sation and nitrification rates. This may have been because clay limited the diffusion of partially decomposed organics away from the decomposing micro bial population, thereby helping to facilitate more complete decomposition of the organic material. Texture had very little influence on the nitrifica tion of urea-derived ammonium.