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Physical and biological soil stabilities (i.e. resistance and resilience) were measured on a range of arable farms across eastern Scotland under a range of management practices, with the objective of using a geographically restricted set of soils under similar land use to detect any underlying associations between soil stability, management factors and soil properties. Data were analysed using a combination of a stepwise fixed effects model selection within a linear mixed-model framework ( LMM) and neural network analysis using a Kohonen self-organising map ( KSOM). In general, physical and biological measures of stability were associated with both physical and biological soil properties, particularly bulk density, water retention characteristics, soil carbon and bacterial community structure. A strength of KSOM is its ability to fit more flexible models than the linear relationships of LMM. However, a weakness is that it does not have the ability of LMM to model the sampling design, which is likely to lead to overstating statistical significance. Consequently, KSOM identified more significant associations between soil properties and stability than LMM, while the latter identified significant associations at the between-farm level. The high-level land management decisions of farm type (conventional, organic, integrated), crop type or underlying soil type were not associated with stability at this regional scale, thus indicating that the effects of different management practices between farms were overridden by the soil properties on each farm. Management decisions on improving soil stability therefore need to be taken at the individual field scale. [ABSTRACT FROM AUTHOR]

Attention is being paid to the use of different tillage regimes as a means of retaining soil organic carbon ( SOC) and sequestering more SOC. Alongside earlier measurements of total SOC stocks under different tillage regimes, we have examined the distribution of nitrogen ( N), microbial activity and the structure of the soil bacterial community from differently tilled plots under continuous barley. The plots were established 5 yr before sampling and have been maintained annually under conventional tillage ( CT; moldboard ploughing to 20 cm and disking), deep ploughing ( DP; ploughing to 40 cm and disking), minimum tillage ( MT; disking to 7 cm) or zero tillage ( ZT). Our earlier work showed there was no difference in SOC contents down to 60-cm depth between the treatments, but now we report that there were significant differences in the total N and active microbial biomass (substrate-induced respiration) contents of the same soils. The N contents of the CT, DP and MT treatments were not significantly different, but the ZT contained significantly more N, indicating either greater N retention under the ZT treatment or preferential loss from the more intensively tilled treatments, or a combination of both. The microbial biomass content was greater for the CT and DP treatments than for the MT and ZT treatments, indicating greater sensitivity to treatment effects of the microbial biomass pool than the total C pool, consistent with its more dynamic nature. Terminal restriction fragment length polymorphism ( T- RFLP) analyses of the soil bacteria DNA (a method of assessing the bacterial community structure) enabled the samples to be distinguished both according to SOC content, which is to be expected, and to tillage regime with the greatest differences in community structure occurring in the ZT treatment and the least in DP and CT treatments, reflecting the degree of homogenization or disturbance resulting from tillage. [ABSTRACT FROM AUTHOR]

It seems possible that soil potential nitrification rates (PNRs) are determined by the size and structure of both the eubacterial and nitrifier populations. We have examined this possibility by comparing the structure of the eubacterial community with the subcomponents and dynamics of the ammonium-oxidizer population, within and between three arable fields. PNRs were significantly different between the three fields and also showed significantly different temporal patterns within each field. The use of eubacterial primers in polymerase chain reaction–denaturing gel gradient electrophoresis (PCR–DGGE) analyses indicated that the bacterial community structure in each field was significantly different from that in the others, and that the bacterial components of these communities changed with time through the season. In contrast, PCR–DGGE analyses specific to ammonium oxidizers suggested that the populations in all three fields were similar in types and did not vary with time. Competitive PCR suggested that there were large and biologically significant differences in the size of the nitrifier population between the three fields, but that within each individual field populations did not change over time. Sequencing identified the ammonium oxidizers in the Nitrosospira spp. cluster. There was also no relation between the size of the nitrifier populations and PNR. Functional expression, as PNR, apparently responded to changes in eubacterial community structure. [ABSTRACT FROM AUTHOR]

An accepted view, that unless nitrate concentrations in the soil solution are very low (e.g. below 0.1-0.2 molm-3) the growth of high-yielding crops is not limited by the availability of nitrogen, is challenged. Conventional analyses of nutrient supply and demand, based on calculations of apparent inflow rates (uptake rates per unit total root length) are invalid. Apparent inflow rates are inversely proportional to root length. The convention of using total root length grossly over-estimates the fraction of the root system active in nutrient uptake. Consequently, inflow rates based on total root lengths underestimate the true values, indicating unrealistically low nutrient concentration differentials between bulk soil and root surfaces required to drive uptake. An alternative method of analysis is suggested. This is based on total nutrient uptake rather than on inflow rate. Measurements of the former do not depend on estimates of active root length and can be made directly and reliably. The method was applied to data obtained from a pot experiment using spring wheat (Triticum aestivum L., cv. Wembley) grown in soil without nitrogen fertilizer (N0) or with nitrogen fertilizer equivalent to 200 kg N ha-1 (N+). Soil nitrate concentrations calculated using the conventional method based on total root length, suggested that any increases in concentration above those measured in the N0 treatment should not have resulted in increased uptake and growth. However, the N+ plants were always bigger than those in the N0 treatment, refuting this suggestion. Theoretical uptakes of nitrogen (calculated initially on the basis of a fully active root system) were adjusted, by reducing the effective root length incrementally, until the theoretical uptake matched the measured net uptake of nitrogen. The mean fractions of the root systems likely to have been involved in nitrate uptake were 11% and 3.5% of the total lengths of root in the N0 and N+ treatments, respectively. [ABSTRACT FROM AUTHOR]