The influence of the soil matrix on nitrogen mineralisation and nitrification. I. Spatial variation and a hierarchy of soil properties

Abstract

Natural heterogeneity of soil properties was used to explore their influence on nitrogen (N) mineralisation and nitrification in undisturbed small soil volumes (soil cells; c. 1 · 7 cm³ ) sampled from a small field plot (2 m by 3 m). Soil cells (840) were randomly ascribed to 1 of 6 treatments in which soils were retained continuously moist (M₁₀ and M₃₀ treatments) and amended with organic N from clover (Cl₁₀ and Cl₃₀ treatments), dried and rewetted (DW₁₀), or treated with urea (Ur₁₀) (subscripts indicate soil incubation at matric potential - 10 or - 30 kPa). After 20 days of incubation at 24C, each soil cell was analysed for NO^-₃ -N, NH + 4 -N, pH, bulk density (BD), volumetric water content (&thgr;v), water content at - 490 kPa (&thgr;v490), and pH buffer capacity (pHBC). On 25 soil cells from each treatment, % clay, % silt, % sand, total N (% N), organic carbon (% C), and 7 cations and anions were also determined.

Net N mineralisation and net nitrification occurred in all treatments, and the total mineral N at the end of the incubation was 497, 81, 73, 31, 27, and 31 µg N/g in the Ur₁₀ Cl₁₀, Cl₃₀, M₁₀, M₃₀, and DW₁₀ treatments, respectively. Net N mineralisation in the M₃₀ treatment was 84% of that in the M₁₀ treatment, and net N mineralisation in the Cl₃₀ treatment was 86% of that in the Cl₁₀ treatment. Fluctuations in soil pH varied markedly between treatments and over time, and it was apparent that alkaline processes were occurring in all soil cells. The heterogeneity between soil samples was substantial for all of the soil variables.

Soil variables were classified in a hierarchy from the least to the most fundamental based on their stability through time. This ranking provides a conceptual tool for understanding interrelationships between soil properties and for interpreting results of regression analyses. The sampling approach adopted in this study was designed to harness the natural heterogeneity of soil properties in the small field site while keeping other properties and environmental factors, that usually vary over larger distances, constant. Both the extent of heterogeneity of soil properties and the nature of their correlations with NO^-₃ -N suggested that this technique would be useful in the exploration of how soil properties influence N mineralisation and nitrification.