P.C. Brookes

Phosphorus Solubilization and Potential Transfer to Surface Waters from the Soil Microbial Biomass Following Drying–Rewetting and Freezing–Thawing

Abstract Drying–rewetting and freezing–thawing are two of the most common forms of abiotic perturbations experienced by soils, and can result in the solubilization of phosphorus (P). There is increasing interest in one particular component of soil P that may be especially susceptible to such stresses: the soil microbial biomass. We examine the evidence for the soil microbial biomass acting as a significant source of P in soils and surface waters by studying the literature on the processes responsible for its solubilization and transfer, resulting from abiotic perturbations. These perturbations have been shown to kill up to circa 70% of the total microbial biomass in some soils, and in some cases nearly all the additional P solubilized has been attributed to the microbial biomass. The degree to which the soil microbial biomass is affected by abiotic perturbations is highly dependent upon many variables, not the least degree, duration, and temporal patterns of stress, as well as the soil type. It is hypothesized that while abiotic perturbations can solubilize large quantities of P from the soil microbial biomass in some soils, only a small proportion is likely to find its way from the soil to surface waters. This is not to say that this small proportion is not significant with regard to surface water quality and nutrient loss from the soil, and may become more prevalent under future climatic change. We conclude that it is likely that only extreme conditions will elicit large responses with regard to the solubilization and transfer of phosphorus to surface waters.

Effect of land use on the abundance and diversity of autotrophic bacteria as measured by ribulose-1,5-biphosphate carboxylase/oxygenase (RubisCO) large subunit gene abundance in soils

Elucidating the biodiversity of CO2-assimilating bacterial communities under different land uses is critical for establishing an integrated view of the carbon sequestration in agricultural systems. We therefore determined the abundance and diversity of CO2 assimilating bacteria using terminal restriction fragment length polymorphism and quantitative PCR of the cbbL gene (which encodes ribulose-1,5-biphosphate carboxylase/oxygenase). These analyses used agricultural soils collected from a long-term experiment (Pantang Agroecosystem) in subtropical China. Soils under three typical land uses, i.e., rice–rice (RR), upland crop (UC), and paddy rice–upland crop rotation (PU), were selected. The abundance of bacterial cbbL (0.04 to 1.25 × 108 copies g−1 soil) and 16S rDNA genes (0.05–3.00 × 1010 copies g−1 soil) were determined in these soils. They generally followed the trend RR > PU > UC. The cbbL-containing bacterial communities were dominated by facultative autotrophic bacteria such as Mycobacterium sp., Rhodopseudomonas palustris, Bradyrhizobium japonicum, Ralstonia eutropha, and Alcaligenes eutrophus. Additionally, the cbbL-containing bacterial community composition in RR soil differed from that in upland crop and paddy rice–upland crop rotations soils. Soil organic matter was the most highly statistically significant factor which positively influenced the size of the cbbL-containing population. The RR management produced the greatest abundance and diversity of cbbL-containing bacteria. These results offer new insights into the importance of microbial autotrophic CO2 fixation in soil C cycling.

How Important Is Microbial Biodiversity in Controlling the Mineralisation of Soil Organic Matter

Nearly all soil organic matter is extensively humified, with some fractions existing for more than 1,000 years. Soil microorganisms are surrounded by about 50 times their mass of soil organic matter but can only metabolise it very slowly (‘basal mineralisation’ rate). Here, we show that the rate-limiting step in soil organic matter mineralisation is independent of microbial biomass size, community structure or activity. We suggest that the rate-limiting step is governed by abiological processes (the ‘regulatory gate’ hypothesis). This has significant implications for our understanding of carbon mineralisation in soils and the role of soil microorganisms in the global carbon cycle.