Substrate inputs and pH as factors controlling microbial biomass, activity and community structure in an arable soil

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Abstract

Our aim was to determine whether the smaller biomasses generally found in low pH compared to high pH arable soils under similar management are due principally to the decreased inputs of substrate or whether some factor(s) associated with pH are also important. This was tested in a soil incubation experiment using wheat straw as substrate and soils of different pHs (8.09, 6.61, 4.65 and 4.17). Microbial biomass ninhydrin-N, and microbial community structure evaluated by phospholipid fatty acids (PLFAs), were measured at 0 (control soil only), 5, 25 and 50 days and CO2 evolution up to 100 days. Straw addition increased biomass ninhydrin-N, CO2 evolution and total PLFA concentrations at all soil pH values. The positive effect of straw addition on biomass ninhydrin-N was less in soils of pH 4.17 and 4.65. Similarly total PLFA concentrations were smallest at the lowest pH. This indicated that there is a direct pH effect as well as effects related to different substrate availabilities on microbial biomass and community structure. In the control soils, the fatty acids 16:1ω5, 16:1ω7c, 18:1ω7c&9t and i17:0 had significant and positive linear relationships with soil pH. In contrast, the fatty acids i15:0, a15:0, i16:0 and br17:0, 16:02OH, 18:2ω6,9, 17:0, 19:0, 17:0c9,10 and 19:0c9,10 were greatest in control soils at the lowest pHs. In soils given straw, the fatty acids 16:1ω5, 16:1ω7c, 15:0 and 18:0 had significant and positive linear relationships with pH, but the concentration of the monounsaturated 18:1ω9 PLFA decreased at the highest pHs. The PLFA profiles indicative of Gram-positive bacteria were more abundant than Gram-negative ones at the lowest pH in control soils, but in soils given straw these trends were reversed. In contrast, straw addition changed the microbial community structures least at pH 6.61. The ratio: [fungal PLFA 18:2w6,9]/[total PLFAs indicative of bacteria] indicated that fungal PLFAs were more dominant in the microbial communities of the lowest pH soil. In summary, this work shows that soil pH has marked effects on microbial biomass, community structure, and response to substrate addition.

Introduction

The effects of soil pH on specific microorganisms, or on the whole microbial biomass, microbial activity and, more recently microbial community structure, have previously been investigated (Anderson, 1998, Bååth and Anderson, 2003). Most studies on the effects of soil pH on microbial community structure have focused on forest soils (e.g., Frostegård et al., 1993a, Bååth et al., 1995, Blagodatskaya and Anderson, 1998, Pennanen, 2001, Bååth and Anderson, 2003). These different studies have demonstrated that soil pH and substrate availability are both important factors in determining the microbial community structure. For example, Frostegård et al. (1993a) monitored shifts in the microbial communities in soils from two different coniferous forests treated with lime or wood-ash. Five to six years later, changes in PLFA patterns indicated that the increase in pH caused a shift in the bacterial community to more Gram-negative and fewer Gram-positive bacteria, while the amount of fungi was unaffected. Bardgett et al. (1996) considered that fungal growth was influenced more by the input of organic substrate than by soil pH. They worked with four grassland soils from a long-term experiment but with a pH range possibly not wide enough to draw general conclusions (pH 4.5–5.4). Blagodatskaya and Anderson (1998) studied 40 forest soils with a much wider pH range (pH 3–6) and reported that, besides pH as the major controlling factor, substrate quality (as reflected by forest type) also influenced the fungal–bacterial ratio. In addition, a pH and substrate interaction occurred where the magnitude of the substrate effect was dependent on the prevailing pH.

Here, we report results from the Hoosfield Acid Strip, a site adjacent to the long-term (or classical) Hoosfield arable field experiments at Rothamsted Research, UK. Soil from the Hoosfield acid strip has a wide range of pH values (from about 8.3–3.7) which form an extremely uniform pH gradient, as a result of a single large application of chalk during the 19th century. It has not received any other amendment including chemical or organic fertilizer since then (Aciego Pietri and Brookes, 2008). Our aims were to test whether changes in microbial biomass, microbial activity and microbial community structure are directly caused by changing pH, or due to changing substrate input, or both. This is a difficult question to answer directly. Generally, the effects of decreasing pH and decreasing substrate inputs are confounded as plant inputs, especially in arable situations, often decline sharply below about pH 5. We attempted to separate these effects in soils of decreasing pH taken along the pH gradient of the Hoosfield Acid Strip. Accordingly the soils, at pHs 8.09, 6.61, 4.65 and 4.17, were incubated under laboratory conditions with or without added wheat straw. Microbial biomass ninhydrin-N, microbial activity as CO2-C evolved and Phospholipid Fatty Acids (PLFAs) were determined. In this way, by supplying additional substrate to low pH soils, it was hoped that we could separate these two main effects.

Section snippets

Sampling and preparation of soils

Soil sampling was carried out at four core positions with different pHs along the Hoosfield Acid Strip. Full details of the pH gradient and how it was formed, more than 100 years ago, are given by Aciego Pietri and Brookes (2008). At each sampling position, twenty cores, each five cm diameter (0–23 cm depth) were taken across the strip and bulked in the field. All the soil samples were sieved moist separately (<2 mm) and their dry matter contents determined (105 °C, 24 h).

Soil treatments

The soils, sampled as

Microbial biomass

Soil pH values were measured at 0, 5, 25 and 50 days after incubation. All pH values decreased during this incubation period, from pHs 8.09, 6.61, 4.65 and 4.17 to 7.65, 5.67, 4.28 and 3.85, respectively, in the straw and fertilizer treatment. We assume that this was due to the addition of N as NH4.

Biomass ninhydrin-N in the control soils changed little with incubation within or between soils (Fig. 1b). The smallest ninhydrin-N concentrations were at pH 4.17 and the highest at pH 8.09

Changes in total biomass and CO2-C evolved due to straw addition

Ninhydrin N can serve as an estimate of biomass because it is very strongly correlated with biomass C (Amato and Ladd, 1988, Joergensen and Brookes, 1990). Straw addition increased ninhydrin-N at all pHs compared to the control. The increase in ninhydrin N following straw addition declined with time but biomass ninhydrin N in the straw amended soils remained larger than biomass ninhydrin N in the controls throughout. The absolute increases in biomass ninhydrin N (i.e. biomass ninhydrin N in

Acknowledgements

We thank P.R. Poulton for helpful discussion. We thank Daniel Abaye for technical advice in the PLFA analyses. Rothamsted Research receives grant aided support from the Biotechnological and Biological Sciences Research Council. We also thank the Council of Scientific and Humanistic Development (CDCH) of the Central University of Venezuela for financial support. Finally we thank three anonymous referees for their patience and for their many helpful suggestions for improving the manuscript.

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