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

Abstract

Small undisturbed soil volumes (c. 1·7 cm3) were collected from the surface of a small field plot. Soil volumes were treated with clover-derived substrate, dried and rewetted, or retained continuously moist from the field. These soil volumes were then incubated for 20 days at a matric water potential of either –10 or –30 kPa. At the end of the incubation the soil was analysed for volumetric water content (θv), NO-3 -N, NH+4 -N, total N (%N), and percentages of sand, silt, and clay. The texture terms were included in linear regression models, together with %N and θv as predictors of N mineralisation and nitrification. Clay and sand were often observed to have a significant influence on N mineralisation and nitrification, but silt rarely appeared to influence these processes. In soils retained continuously moist, %clay had a negative relationship with N mineralisation and nitrification, but this relationship was positive 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 attack. 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 therefore 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 resident in larger pores (10–30 µm neck diameter) may not be as strongly regulated by clay as that resident in smaller pores. When soils were treated with clover-derived substrate, clay had a positive relationship with N mineralisation and nitrification rates. This may have been because clay limited the diffusion of partially decomposed organics away from the decomposing microbial population, thereby helping to facilitate more complete decomposition of the organic material. Texture had very little influence on the nitrification of urea-derived ammonium.