Relationships between soil pH and microbial properties in a UK arable soil

Abstract

Effects of changing pH along a natural continuous gradient of a UK silty-loam soil were investigated. The site was a 200 m soil transect of the Hoosfield acid strip (Rothamsted Research, UK) which has grown continuous barley for more than 100 years. This experiment provides a remarkably uniform soil pH gradient, ranging from about pH 8.3 to 3.7. Soil total and organic C and the ratio: (soil organic C)/(soil total N) decreased due to decreasing plant C inputs as the soil pH declined. As expected, the CaCO₃ concentration was greatest at very high pH values (pH > 7.5). In contrast, extractable Al concentrations increased linearly (R² = 0.94, p < 0.001) from below about pH 5.4, while extractable Mn concentrations were largest at pH 4.4 and decreased at lower pHs. Biomass C and biomass ninhydrin-N were greatest above pH 7. There were statistically significant relationships between soil pH and biomass C (R² = 0.80, p < 0.001), biomass ninhydrin-N (R² = 0.90, p < 0.001), organic C (R² = 0.83, p < 0.001) and total N (R² = 0.83, p < 0.001), confirming the importance of soil organic matter and pH in stimulating microbial biomass growth. Soil CO₂ evolution increased as pH increased (R² = 0.97, p < 0.001). In contrast, the respiratory quotient (qCO₂) had the greatest values at either end of the pH range. This is almost certainly a response to stress caused by the low p. At the highest pH, both abiotic (from CaCO₃) and biotic Co₂ will be involved so the effects of high pH on biomass activity are confounded. Microbial biomass and microbial activity tended to stabilise at pH values between about 5 and 7 because the differences in organic C, total N and Al concentrations within this pH range were small. This work has established clear relationships between microbial biomass and microbial activity over an extremely wide soil pH range and within a single soil type. In contrast, most other studies have used soils of both different pH and soil type to make similar comparisons. In the latter case, the effects of soil pH on microbial properties are confounded with effects of different soil types, vegetation cover and local climatic conditions.

Introduction

Soil pH affects all chemical, physical and biological soil properties (Brady and Weil, 2002). The effect of soil pH on specified micro-organisms, on soil microbial biomass, microbial activity and, more recently, on microbial community structure, have been investigated previously. Wardle (1992) concluded that soil pH is probably at least as important as soil C and N concentrations in influencing the size of the microbial biomass. Soil pH also affects organic C solubility (Andersson et al., 2000) and increases the availability of biologically toxic Al with decreasing pH (Flis et al., 1993). This, in turn, affects microbial community structure (e.g. Anderson, 1998, Zelles, 1999, Marstorp et al., 2000) and changes in microbial activity (Bååth and Anderson, 2003).

Some researchers have studied soils with natural pH differences (Anderson and Joergensen, 1997). Others have studied soils in which the pH was changed through anthropogenic intervention, e.g. liming, ash application, alkaline or acidifying pollution (e.g. Anderson, 1998, Thirukkumaran and Parkinson, 2000, Chagnon et al., 2001). Such changes may be short term. Thus, Kemmitt et al. (2005) acidified grassland and arable field plots and showed that the resulting acidification lowered nitrate in the grassland plots in both winter and spring and in cereal plots in spring only during the next 2 years.

Most studies on the effects of soil pH on microbial processes have focused on forest soils (Bååth et al., 1992, Bååth et al., 1995, Frostegård et al., 1993, Blagodatskaya and Anderson, 1998, Pennanen, 2001, Bååth and Anderson, 2003). Less work has been done on arable, grassland or other soils (Bardgett et al., 2001, Arao, 1999, Schutter and Fuhrmann, 2001). In most of the above, variations in soil pH were usually produced by applications of lime or burnt coniferous residues to different soils (Frostegård et al., 1993, Bååth et al., 1995), coal fly ash addition on field plots (Schutter and Fuhrmann, 2001), or industrial pollution with alkaline dust (Bååth et al., 1992).

In other investigations, variations in soil pH were natural, due to chemical characteristics of different types of soils (Blagodatskaya and Anderson, 1998, Arao, 1999, Pennanen, 2001, Bååth and Anderson, 2003). These different studies have demonstrated that soil pH and substrate availability are important factors in determining soil microbial activities. However, the confounding effects of soil type and varying managements remain.

Here, we report the effects of changing soil pH on soil microbial biomass and microbial activity in soils from the Hoosfield acid strip, one of the long term (or Classical) arable field experiments at Rothamsted Research, UK. The Hoosfield acid strip is a perfect site to carry out this investigation. It has an extremely uniform pH gradient (range about 8.3–3.7) along a single soil type, as a result of chalk applied unevenly in the 19th century. It therefore probably is the oldest artificially produced pH gradient in the world.