Effect of soil type on the N immobilization by decomposing straw at the South of Portugal

Abstract

In many agricultural systems in the south of Portugal leaving crop residues on the soil surface after cropping is a common practice. During autumn these residues are either burned or incorporated in the soil. The latter is preferable as the residues are rich in C and consequently promoting mineral nitrogen immobilization both from the soil organic matter and residues mineralization. This reducing nitrate potential high loss by leaching caused by the high rainfall, which usually, occurs in autumn and winter in southern Portugal.

It is necessary to find ways to reduce N loss by leaching, understand how different agricultural systems influence this process and identify ways to retain N for a longer period in the system soil-plant. One possibility is the synchronization of the N mineralization with either their consumption by the plants, through N immobilization in the residues rich in C.

The goal of this work was to study the effect of two soils of contrasting fertility in N immobilization and subsequent mineralization, namely a sandy-loam soil (Pmg) in Évora and a clay soil (Bvc) in Beja.

Wheat straw was incubated in nylon net bags for one year, with sampling periods of 1, 4, 8 and 12 months. The trial was composed of 3 treatments: without N, with addition of ¹⁵NH4NO3 and with addition of NH4¹⁵NO3. N and C total concentration in the straw and soil was determined and ¹⁵N abundance was measured. The remaining proportion of the initial quantity of straw was incubated and the residuals total N concentration and ¹⁵N immobilization was compared.

During the first month as ¹⁵N was being incorporated in the soil the residual mass and total N content decreased. Subsequently net immobilization started to occur and total N content increased, though with different rates according to the soil type. In Bvc soil ¹⁵ NO3 highest incorporation was observed while in Pmg soil the same was observed with ¹⁵NH4.