Increases in Potentially Mineralizable and Dissolved Organic Nitrogen in a Sandy Surface Soil Fertilized with Nitrogen are Greater with Lupin than Wheat Residues

Abstract

Maintaining adequate nitrogen (N) supply to dryland cereal crops remains a critical management issue for optimizing productivity and N-use efficiency. Combinations of crop residue and fertilizer-N inputs can be used to manipulate soil-N supply factors at critical stages of plant N demand. The aim of this study was to understand the effects of residue and N fertilizer combinations on soil-N supply capacity to wheat (Triticum aestivum L.) crop plants under field conditions. Residue decomposition and N release in the field during a fallow period and the subsequent wheat growing season were quantified using litterbags containing wheat or lupin residues placed in the top 1–2 cm of soil. During the growing season, surface (0–10 cm depth) soil-N capacity was assessed in relation to key wheat growth stages by measuring labile organic and inorganic soil-N pools under a combination of different residue (removed, wheat, or lupin residues) and fertilizer-N (0, 20, or 40 kg N ha−1) treatments applied at sowing. Around half of the N release by residues occurred in the fallow period. Lupin residues provided a greater N release via net mineralization and dissolved organic N (DON) early in the growing season (up to tillering). As the crop developed, lupin residues had a higher surface soil potentially mineralizable N (PMN) and microbial biomass N (MBN) compared with wheat or removed residues. It was the combination of lupin residues with fertilizer-N that had the highest overall amount of surface soil DON and PMN between seedling and tillering. This study showed that the fallow is an important component of the N cycle in these systems as 40–50% of crop residue N release occurred during this period. However, this release represented less than 15% of subsequent wheat crop N uptake, which highlights the importance of N cycling from recent residues and pre-existing organic matter during the crop growing season for meeting crop N demand. Lupin residues and fertilizer-N inputs, both independently and together, increased soil-N supply capacity at critical crop growth stages in this low-rainfall sandy soil environment.