Andrew S. Gregory

The effect of soil strength on the yield of wheat

Although it is well-known that high soil strength is a constraint to root and shoot growth, it is not clear to what extent soil strength is the main physical stress that limits crop growth and yield. This is partly because it is difficult to separate the effects of soil drying and high soil strength, which tend to occur together. The aim of this paper is to test the hypothesis that for two different soil types, yield is closely related to soil strength irrespective of difference in soil water status and soil structure. Winter (Triticum aestivum L., cv. Hereward) and spring wheat (cv. Paragon) were grown in the field on two soils, which had very different physical characteristics. One was loamy sand and the other sandy clay loam; compaction and loosening treatments were applied in a fully factorial design to both. Crop growth and yield, carbon isotope discrimination, soil strength, water status, soil structure and hydraulic properties were measured. The results showed that irrespective of differences in soil type, structure and water status, soil strength gave a good prediction of crop yield. Comparison with previous data led to the conclusion that, irrespective of whether it was due to drying or compaction (poor soil management), soil strength appeared to be an important stress that limits crop productivity.

Estimating soil strength in the rooting zone of wheat

When roots abstract water thus drying the soil, crop growth may be reduced by increasing strength of soil as well as the lack of water. Strong soil impedes root growth, restricting access to deeper water. As a result, there is a need to estimate soil strength in order to model crop response to dry soil correctly. The strength of soil can be routinely assessed with a penetrometer but measurements are time consuming and hard work to acquire at the frequency required to understand soil-water-plant relations. To make progress, a published relationship that derives penetrometer pressure from both water relations in soil and density was improved to take account of the effects of depth including the friction that results from the increasing hydrostatic pressure. These relationships were then incorporated into an agroecosystem model so that the dynamics of strong soil and its effect on wheat could be simulated. The combined model requires the moisture release curve (but this can be derived from other commonly-measured soil properties), daily rainfall, temperature, and potential evaporation and the agronomy of the crop. Modelled values of penetrometer pressure were simulated well compared with measured values in artificially strengthened (compacted) and weakened (irrigated) soils. Simulations of the strength of soil and the matric potential before anthesis are compared with measured total dry-matter yields of winter wheat in experimental fields. The results lend weight to the hypothesis that wheat yield is limited by the strength of soil in the field and that soil strength, rather than soil matric potential, better explains differences between soils.

Isotope fractionation factors controlling isotopocule signatures of soil-emitted N₂O produced by denitrification processes of various rates.

RATIONALE This study aimed (i) to determine the isotopic fractionation factors associated with N2O production and reduction during soil denitrification and (ii) to help specify the factors controlling the magnitude of the isotope effects. For the first time the isotope effects of denitrification were determined in an experiment under oxic atmosphere and using a novel approach where N2O production and reduction occurred simultaneously. METHODS Soil incubations were performed under a He/O2 atmosphere and the denitrification product ratio [N2O/(N2 + N2O)] was determined by direct measurement of N2 and N2O fluxes. N2O isotopocules were analyzed by mass spectrometry to determine δ(18)O, δ(15)N and (15)N site preference within the linear N2O molecule (SP). An isotopic model was applied for the simultaneous determination of net isotope effects (η) of both N2O production and reduction, taking into account emissions from two distinct soil pools. RESULTS A clear relationship was observed between (15)N and (18)O isotope effects during N2O production and denitrification rates. For N2O reduction, diverse isotope effects were observed for the two distinct soil pools characterized by different product ratios. For moderate product ratios (from 0.1 to 1.0) the range of isotope effects given by previous studies was confirmed and refined, whereas for very low product ratios (below 0.1) the net isotope effects were much smaller. CONCLUSIONS The fractionation factors associated with denitrification, determined under oxic incubation, are similar to the factors previously determined under anoxic conditions, hence potentially applicable for field studies. However, it was shown that the η(18)O/η(15)N ratios, previously accepted as typical for N2O reduction processes (i.e., higher than 2), are not valid for all conditions.

Long-term management changes topsoil and subsoil organic carbon and nitrogen dynamics in a temperate agricultural system

Summary Soil organic carbon (SOC) and nitrogen (N) contents are controlled partly by plant inputs that can be manipulated in agricultural systems. Although SOC and N pools occur mainly in the topsoil (upper 0.30 m), there are often substantial pools in the subsoil that are commonly assumed to be stable. We tested the hypothesis that contrasting long‐term management systems change the dynamics of SOC and N in the topsoil and subsoil (to 0.75 m) under temperate conditions. We used an established field experiment in the UK where control grassland was changed to arable (59 years before) and bare fallow (49 years before) systems. Losses of SOC and N were 65 and 61% under arable and 78 and 74% under fallow, respectively, in the upper 0.15 m when compared with the grass land soil, whereas at 0.3–0.6‐m depth losses under arable and fallow were 41 and 22% and 52 and 35%, respectively. The stable isotopes 13C and 15N showed the effects of different treatments. Concentrations of long‐chain n‐alkanes C27, C29 and C31 were greater in soil under grass than under arable and fallow. The dynamics of SOC and N changed in both topsoil and subsoil on a decadal time‐scale because of changes in the balance between inputs and turnover in perennial and annual systems. Isotopic and geochemical analyses suggested that fresh inputs and decomposition processes occur in the subsoil. There is a need to monitor and predict long‐term changes in soil properties in the whole soil profile if soil is to be managed sustainably. Highlights Land‐use change affects soil organic carbon and nitrogen, but usually the topsoil only is considered. Grassland cultivated to arable and fallow lost 13–78% SOC and N to 0.6 m depth within decades. Isotopic and biomarker analyses suggested changes in delivery and turnover of plant‐derived inputs. The full soil profile must be considered to assess soil quality and sustainability.

A review of the impacts of degradation threats on soil properties in the UK

Abstract National governments are becoming increasingly aware of the importance of their soil resources and are shaping strategies accordingly. Implicit in any such strategy is that degradation threats and their potential effect on important soil properties and functions are defined and understood. In this paper, we aimed to review the principal degradation threats on important soil properties in the UK, seeking quantitative data where possible. Soil erosion results in the removal of important topsoil and, with it, nutrients, C and porosity. A decline in soil organic matter principally affects soil biological and microbiological properties, but also impacts on soil physical properties because of the link with soil structure. Soil contamination affects soil chemical properties, affecting nutrient availability and degrading microbial properties, whilst soil compaction degrades the soil pore network. Soil sealing removes the link between the soil and most of the ‘spheres’, significantly affecting hydrological and microbial functions, and soils on re‐developed brownfield sites are typically degraded in most soil properties. Having synthesized the literature on the impact on soil properties, we discuss potential subsequent impacts on the important soil functions, including food and fibre production, storage of water and C, support for biodiversity, and protection of cultural and archaeological heritage. Looking forward, we suggest a twin approach of field‐based monitoring supported by controlled laboratory experimentation to improve our mechanistic understanding of soils. This would enable us to better predict future impacts of degradation processes, including climate change, on soil properties and functions so that we may manage soil resources sustainably.

Short-term soil development in a clay soil-forming material used as a landfill restoration cap: a case study

Time is one of the principal factors affecting soil formation, as it controls the extent to which natural processes convert soil-forming materials (SFMs) into media suitable for sustaining vegetation growth. This becomes especially important for land restoration projects where SFMs have to be used in the absence of soil material. This paper examines short-term soil development in a clay SFM used as a landfill restoration cap by characterizing SFMs on an operational site that were restored at different points in time. SFMs that had been in situ for ten years since restoration retained less water at near-saturation and had a greater degree of aggregation. Organic matter (OM) and microbial biomass contents also tended to be greater in older-restored SFMs. Vegetation type appeared to control nitrogen (N) and respiration, and the sowing of legumes may be especially important where SFMs are used as soil substitutes. The results suggest that soil development in the SFM was indeed controlled by time, especially when compared with recent allied studies examining the use of soil amendments at the same site. The enhancement of the biological status proceeded at a faster rate than structure improvement. Within a ten-year period of restoration, a soil material capable of supporting acceptable vegetation growth appeared to have developed on the site. Nevertheless, land restoration targets should be set with the important soil-forming factor of time in mind.