Effects of incorporated and mulched organic amendments on soil nutrient availability, microbial activity and plant growth

Abstract

Little is known about the effects of repeated addition of mixes of organic materials differing in C/N ratio and decomposability on nutrient availability, microbial activity in soils and plant growth. Further, there is little information about the effects of mulches differing in C/N ratio and decomposability placed on soil mixed with plant residues. The aim of the thesis is to determine the effects of organic amendments on nutrient availability, microbial activities and plant growth. The first two incubation experiments were carried out to investigate effects of amendment rate, order and frequency on soil nutrient availability and microbial biomass. In these experiments, residues differing in C/N ratio (high, mature wheat straw C/N 82, H) and (low, young faba bean, C/N 9, L) were added into soil sequentially (H-L or L-H) or as 1:1 mixes (HL). Residues or their mix were added two and four times in the first experiment and two, four and eight times in the second experiment. In both experiments, with repeated addition of H and L, N availability and microbial biomass N were influenced by residue rate and order. From these two experiments it can be concluded that N availability remains stable with frequent addition of residues with different C/N ratio whereas it strongly fluctuates when large amounts of residues with different C/N ratio are added. The third incubation and first pot experiment were conducted to assess the effect of mixing organic materials differing in C/N ratio and decomposition stage on soil nutrient availability, microbial activity and plant growth. In these experiments, soil was amended with young faba bean shoots (C/N 9), sheep manure (C/N 6) and mature wheat straw (C/N 82) either individually or as 25:75, 50:50 and 75:25 mixes. In the incubation experiment, the effect on N availability, microbial biomass N and respiration was lower with sheep manure than with faba bean which can be explained by highly decomposed state of sheep manure. Mixing sheep manure or wheat with ≥50% faba bean maintained higher N availability than sheep manure or wheat alone, but prevented the rapid N release that occurred after the addition of sole faba bean. In the pot experiment, in mixes of faba bean with 25% wheat or sheep manure, plant growth and N uptake were similar or higher than the control. It can be concluded that in these mixes, plant roots access organic particles releasing N and thereby compensate low N availability. To investigate the effects of mulching organic materials differing in C/N ratio and decomposition stage with or without high C/N residues mixed into the soil on soil nutrient availability, microbial activity and plant growth, the fourth incubation and second pot experiment were conducted. In these two experiments, soil either had no amendments, only mature wheat straw incorporated, mulch (mature wheat straw, young faba bean shoot or sheep manure) over wheat straw, or mulch only. In both experiments, wheat straw incorporation under rapidly decomposing low C/N mulch reduced the risk of N leaching from the mulch due to greater microbial N uptake compared to mulch alone.