Effect of crop residue quality on phosphorus pools in the detritusphere and P uptake by wheat

Abstract

Little is known about the effect of the influence of water availability, crop residue quality and plant growth on phosphorus (P) pools in the detritusphere, the soil adjacent to plant residues. The detritusphere soil was generated in microcosms as described in Ha et al. (2007). The soil at 0-2 mm distance from the surface of soil incubated in PVC caps was collected as the detritusphere soil and used for further experiments. Bioavailable P pools (readily available P pools: CaCl2 and anion exchange P; P bound to soil particles: citrate and HCl P; acid phosphatase and microbial P), available N and microbial N were measured in the detritusphere. The experiment described in Chapter 2 investigated the influence of drying and rewetting on soil P pools in the detritusphere of two crop residues, young faba bean residue (C/P 38) and mature barley straw (C/P 255). The detritusphere and unamended control soils were dried to approximately 5% water holding capacity (WHC) and kept dry for two weeks followed by rapid rewetting to 50% WHC, or maintained at 50% of WHC. Rewetting of dry soils induced a respiration flush and the flush was greater with faba bean than barley. P pools were higher with faba bean than with barley, due to lower C/P ratio of the former. In general, drying and rewetting had little effect on P pools. In Chapter 3, an experiment is described that assessed the influence of soil water availability on P pools in the detritusphere of crop residues. Detritusphere was generated with barley straw (C/P 255) or barley straw mixed with faba bean residue at a 75:25 ratio (C/P 200) in soil at 50% WHC. Water availability in the detritusphere soils was reduced to -0.320 and -1.700 MPa (30% and 10% WHC), or maintained at -0.078 MPa (50% WHC). In the detritusphere of the residue mix, soil respiration, P pools and available N were lower at -1.700 MPa than at -0.078 MPa. However, water availability had little effect in barley detritusphere. The aim of the experiment described in Chapter 4 was to elucidate the effect of soil amendment with inorganic N and P on P pools in the detritusphere of mature barley straw (C/N 95; C/P 255). Addition of inorganic N to soil increased P pools likely due to enhanced mineralisation of native soil organic matter. Barley straw decomposition reduced available P pools in the detritusphere, particularly in soil to which inorganic P was added. In Chapter 5, an experiment was described to determine the influence of a change of residue types on P pools in the detritusphere of crop residues with differing C/P ratios. In the first experiment, after two weeks of incubation at 50% WHC, with young faba bean residue (L) or mature barley straw (H), the residues were replaced with either a H or L, resulting in four residue treatments: high-high (HH), high-low (HL), low-low (LL) or low-high (LH), which were incubated for another 14 days. On day 14, P pools and available N were higher, but MBP and MBN were lower in L than in H. On day 28, P pools and available N followed the order LL>HL>LH>HH, whereas MBN and MBP were highest in HL. The experiment described in Chapter 6 aimed to determine the influence of residue C/P ratio on changes in P pools and N availability in wheat rhizosphere. Pre-germinated wheat seeds were sown in unamended soil or soil amended with two crop residues (young faba bean residue, C/P 38; mature barley straw, C/P 255). After 28 days with faba bean, P uptake in wheat was higher than with barley straw and control. P pools were lower in the interface of wheat rhizosphere and faba bean detritusphere than in detritusphere alone, due to plant uptake. With barley straw, presence of wheat roots had no effect on P pools.