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Weeds represent a significant threat to crop yields and global food security. We analysed data on weed competition from the world's longest running agricultural experiment to ask whether potential yield losses from weeds have increased in response to management and environmental change since the advent of the Green Revolution in the 1960s. On plots where inorganic nitrogen fertiliser has been applied, potential yield losses from weeds have consistently increased since 1969. This was explained by a warming climate, measured as air temperature averaged over the growing season for the weeds, and a shift towards shorter crop cultivars. Weeds also reduced yield proportionally more on plots with higher rates of nitrogen which had higher yields when weeds were controlled; the relative benefit of herbicides was, therefore, proportional to potential crop yield. Reducing yield losses from weed competition is increasingly challenging because of the evolution of herbicide resistance. Our results demonstrate that weeds now represent a greater inherent threat to crop production than before the advent of herbicides and integrated, sustainable solutions to weed management are urgently needed to protect the high yield potential of modern crop genotypes., The world's longest running agricultural field experiment has plots that have never received herbicides. When yields from these plots were compared to those from equivalent plots where weeds are controlled with herbicides, we observed increasing yield losses between 1969 and 2014. This was explained by rising temperatures and the shift towards shorter crop cultivars.

The development of resilient cropping systems with high yield stability is becoming increasingly important due to future climatic and agronomic challenges. Consequently, it is essential to compare the effects of different agronomic management practices, such as cropping sequences and nutrient supply, on the stability of crop yields. Long-term experiments are a valuable resource for investigating these effects, as they provide enough time to accurately estimate stability parameters. The objective of the current study was to compare the effects of different cropping sequencing (#1: continuous v. rotational), fertilization (#2: mineral v. organic) and straw management techniques (in the case of continuous wheat; #3: removal v. incorporation) on the yield stability of winter wheat; yield risk (the probability of yield falling below a threshold yield level) and inter-annual yield variability were used as stability indicators of the effects. Long-term yield data from the Broadbalk Wheat Experiment (Rothamsted, UK) were analysed using a mixed model. Overall, the results showed that rotational cropping combined with sufficient mineral N fertilizer, with or without organic manure, ensured stable wheat yields while reducing yield risk. In contrast, higher yield risks and inter-annual yield variabilities were found in continuous wheat sections with less mineral N fertilizer or with organic manure only.

The link between carbon turnover model pools and measurable carbon fractions is of key interest for initial parameterisation and subsequent validation of dynamic soil carbon models. In this study we performed the established particle-size fractionation of soils from 54 intensively monitored sites in Germany and from archived samples from 5 other long-term experiments in Germany and the United Kingdom. The Rothamsted carbon (RothC) model was then used to compare the measured soil C fractionation from the 54 intensively monitored sites against modelled pools using spin-up equilibrium runs whilst dynamic (non-equilibrium) model runs were performed when comparing data from the long-term experiments. We detected good agreement between measured soil C fractions and modelled pools, indicated by correlation coefficients of 0.73 and 0.81 for the resistant plant material pool (RPM) and 0.91 and 0.94 for the humus pool (HUM) for the intensively monitored and the long-term sites, respectively. Slightly larger errors were detected for the intensively monitored sites together with a bias in the relationship between the RPM pool and particulate organic matter fraction. This bias detected for the intensively monitored sites indicated that the equilibrium assumption for arable agricultural sites, even though under crop cover for at least 50 years, might not be entirely valid. From the relative mean absolute error of 11% for the HUM pool and 26% for the RPM pool of the combined data set we conform that the measured fractions can be used to estimate the RothC model pools in arable soils. Given the magnitude of these errors, however, we rather suggest to apply the fractionation approach instead of using an equilibrium assumption for the RothC initialisation of arable sites.

The sequestration in soil of organic carbon (SOC) derived from atmospheric carbon dioxide (CO2) by replacing arable crops with leys, has been measured over 70 years on a sandy loam soil. The experiment was designed initially to test the effect of leys on the yields of arable crops. A 3-year grazed grass with clover (grass + clover) ley in a 5-year rotation with arable crops increased percentage organic carbon (%OC) in the top 25 cm of the soil from 0.98 to 1.23 in 28 years, but with little further increase during the next 40 years with all-grass leys given fertilizer nitrogen (N). In this second period, OC inputs were balanced by losses, suggesting that about 1.3% OC might be near the equilibrium content for this rotation. Including 3-year lucerne (Medicago sativa) leys had little effect on %OC over 28 years, but after changing to grass + clover leys, %OC increased to 1.24 during the next 40 years. Eight-year leys (all grass with N or grass + clover) in 10-year rotations with arable crops were started in the 1970s, and after three rotations %OC had increased to ca. 1.40 in 2000–2009. Over 70 years, %OC declined from 0.98 to 0.94 in an all-arable rotation with mainly cereals and to 0.82 with more root crops. Applications of 38 t ha−1 farmyard manure (FYM) every fifth year increased %OC by 0.13% by the mid-1960s when applications ceased. Soil treated with FYM still contained 0.10% more OC in 2000–2009. Changes in the amount of OC have been modelled with RothC-26.3 and estimated inputs of C for selected rotations. Little of the OC input during the 70 years has been retained; most was retained in the grazed ley rotation, but 9 t ha−1 only of a total input of 189 t ha−1. In other rotations more than 98% of the total OC input was lost. Despite large losses of C, annual increases in OC of 4‰ are possible on this soil type with the inclusion of grass or grass + clover leys or the application of FYM, but only for a limited period. Such increases in SOC might help to limit increases in atmospheric CO2. Highlights Can leys sequester significant amounts of atmospheric CO2 in SOM and contribute to the 4‰ initiative? Changes in the percentage and amount of OC were measured and modelled over 70 years and OC losses estimated. Three-year grass or grass + clover leys increased %OC, but only to an equilibrium level that was then maintained. Despite large losses, sequestering CO2-C at 4‰ year−1 by growing grass or grass + clover leys is possible.

Traditional methods to assess the soil organic carbon (SOC) content based on soil sampling and analysis are timeconsuming and expensive, and the results are influenced by the sampling design. The aim of this study was toinvestigate the potential of UAS (Unmanned Aerial Systems) multi-spectral imagery (480–1000 nm) for estimatingthe SOC content in bare cultivated soils at a high spatial resolution (12 cm).We performed UAS analysis onthe Hoosfield Spring Barley experiment at Rothamsted (UK) where adjacent plots with distinctly different SOCcontents, due to different long-term management practices, provide a valuable resource to evaluate thisapproach. We acquired images (wavelength: 480–550–670–780–880–1000 nm) at an altitude of 120 m overan area of 2 ha using a multi-spectral camera mounted on an UAS. The high-resolution images captured smallscalevariations at the soil surface (e.g. shadows, tillage andwheels marks). After a projection in newdimensionsby a PCA, we calibrated a support vector machine regression using observations fromconventional soil samplingand SOC measurements. The performance of the calibration had a R2 of 0.98 and a RMSE of 0.17%C. A crossvalidationshowed that the model was robust, with an average R2 of 0.95 and a RMSE of 0.21%. An externalvalidation dataset was used to evaluate the predicted spatial patterns of SOC content and a good fit with anRMSE 0.26%C was obtained. Although this study shows that the methodology has a clear potential for use inprecision agriculture or monitoring important soil properties following changes in management,we also identifyand discuss its limitations and current shortcomings.

Once phosphorus (P) applications stop, it is important to know how long it will take for plant-available P in soil to decline to a particular level for soils with a history of intensive management. The rate of decline in the absence of applied P can be expressed as a half-life, that is the time in years for it to halve relative to the lowest level measured in the soil. This knowledge is essential when planning long-term policies for managing the P status of soils. It is important to farmers who wish to optimize applications of P fertilizers and make better use of accumulated P residues to ensure that plant-available P does not fall below the critical level for the soil and crop because of the risk of losing yield and the inefficient use of other inputs, especially N fertilizer. Lower levels of plant-available P are also desirable for environmental and ecological reasons. Allowing plant-available P to decline to the critical level from unnecessarily high concentrations will reduce the risk from P on eroded soil causing adverse effects of eutrophication in surface water. Low levels of plant-available P are usually considered necessary for establishing species-rich grassland. A method for determining the longer term decline in plant-available soil P from short-term measured values is presented.

The negative effect of increasing atmospheric nitrogen (N) pollution on grassland biodiversity is now incontrovertible. However, the recent introduction of cleaner technologies in the UK has led to reductions in the emissions of nitrogen oxides, with concomitant decreases in N deposition4. The degree to which grassland biodiversity can be expected to ‘bounce back’ in response to these improvements in air quality is uncertain, with a suggestion that long-term chronic N addition may lead to an alternative low biodiversity state5. Here we present evidence from the 160-year-old Park Grass Experiment at Rothamsted Research, UK6, that shows a positive response of biodiversity to reducing N addition from either atmospheric pollution or fertilizers. The proportion of legumes, species richness and diversity increased across the experiment between 1991 and 2012 as both wet and dry N deposition declined. Plots that stopped receiving inorganic N fertilizer in 1989 recovered much of the diversity that had been lost, especially if limed. There was no evidence that chronic N addition has resulted in an alternative low biodiversity state on the Park Grass plots, except where there has been extreme acidification, although it is likely that the recovery of plant communities has been facilitated by the twice-yearly mowing and removal of biomass. This may also explain why a comparable response of plant communities to reduced N inputs has yet to be observed in the wider landscape.

Archived samples from the Park Grass Experiment, established in 1856, were analysed to determine the impacts of long-term phosphate fertiliser applications on arsenic concentrations in soil and herbage. In plots receiving 35 kg P ha −1 annually (+P), topsoil As concentrations almost doubled from an initial value of ∼10 mg kg −1 during 1888–1947 and remained stable thereafter. The phosphate fertilisers used before 1948 contained 401–1575 mg As kg −1 , compared to 1.6–20.3 mg As kg −1 in the later samples. Herbage samples from the +P plots collected during 1888–1947 contained significantly more As than those from the −P plots, but later samples did not differ significantly. Mass-balance calculations show that the increase in soil As can be explained by the As input from P fertiliser applications before 1948. The results demonstrate that the P fertilisers used on the Park Grass Experiment before 1948 caused substantial As contamination of the soil.

Under the increase in atmospheric CO2 during the last century, variable increases in the intrinsic water-use efficiency (Wi), i.e., the ratio between carbon assimilation rate (A) and stomatal conductance (gs), of C3 vegetation have been observed. Here, we ask if long-term nutrient status and especially nitrogen supply have an effect on the CO2 response of Wi in a temperate seminatural C3 grassland. This analysis draws on the long-term trends (1915-2009) in Wi, derived from carbon isotope analysis, of archived hay and herbage from the Park Grass Experiment at Rothamsted (South-East England). Plant samples came from five fertilizer treatments, each with different annual nitrogen (N; 0, 48 or 96 similar to kg similar to ha-1), phosphorus (P; 0 or 35 similar to kg similar to ha-1) and potassium (K; 0 or 225 similar to kg similar to ha-1) applications, with lime as required to maintain soil pH near 7. Carbon isotope discrimination (13?) increased significantly (P < 0.001) on the Control (0.9 parts per thousand per 100 similar to ppm CO2 increase). This trend differed significantly (P < 0.01) from those observed on the fertilized treatments (PK only: 0.4 parts per thousand per 100 similar to ppm CO2 increase, P < 0.001; Low N only, Low N+PK, High N+PK: no significant increase). The 13? trends on fertilized treatments did not differ significantly from each other. However, N status, assessed as N fertilizer supply plus an estimate of biologically fixed N, was negatively related (r2 similar to=similar to 0.88; P < 0.02) to the trend for 13? against CO2. Other indices of N status exhibited similar relationships. Accordingly, the increase in Wi at High N+PK was twice that of the Control (+28% resp. +13% relative to 1915). In addition, the CO2 responsiveness of 13? was related to the grass content of the plant community. This may have been due to the greater CO2 responsiveness of gs in grasses relative to forbs. Thus, the greater CO2 response of grass-rich fertilized swards may be related to effects of nutrient supply on botanical composition.

Results from the UK were reviewed to quantify the impact on climate change mitigation of soil organic carbon (SOC) stocks as a result of (1) a change from conventional to less intensive tillage and (2) addition of organic materials including farm manures, digested biosolids, cereal straw, green manure and paper crumble. The average annual increase in SOC deriving from reduced tillage was 310 kg C ± 180 kg C ha−1 yr−1. Even this accumulation of C is unlikely to be achieved in the UK and northwest Europe because farmers practice rotational tillage. N2O emissions may increase under reduced tillage, counteracting increases in SOC. Addition of biosolids increased SOC (in kg C ha−1 yr−1 t−1 dry solids added) by on average 60 ± 20 (farm manures), 180 ± 24 (digested biosolids), 50 ± 15 (cereal straw), 60 ± 10 (green compost) and an estimated 60 (paper crumble). SOC accumulation declines in long-term experiments (>50 yr) with farm manure applications as a new equilibrium is approached. Biosolids are typically already applied to soil, so increases in SOC cannot be regarded as mitigation. Large increases in SOC were deduced for paper crumble (>6 t C ha−1 yr−1) but outweighed by N2O emissions deriving from additional fertiliser. Compost offers genuine potential for mitigation because application replaces disposal to landfill; it also decreases N2O emission.&nbsp

Accurate characterization of dissolved organic matter (DOM) in soil is vital for understanding its functional signifi cance. In this study, we used ultrafi ltration to determine the molecular weight (MW) distribution of dissolved organic C (DOC), N (DON), and phenolics in two contrasting agricultural grassland soils. This MW fractionation (>100, 10–100, 1–10,and 1 kDa) in both soils. This high-MW fraction contained a signifi cant amount of organic N, however, and was no more enriched in phenolic-containing substances than the low-MW fraction. Our results are consistent with the view that soil solutions contain a diverse size range of organic compounds but that the high-MW fraction does not contain a disproportionate amount of phenolics or N.

Although it has been hypothesized that soluble organic nitrogen (SON) plays a central role in regulating productivity in some terrestrial ecosystems, the factors controlling the size of the SON pool in soil remain poorly understood. Therefore our principal aim in this work was to assess the impact of seven different land use systems (rough and managed grassland, deciduous and coniferous woodland, heathland, wetland and tilled land) on the size of the SON and inorganic N (NO−3, NH+4) pools in the surface soil layer (0–15 cm). After extraction with deionised water, we found that in most cases the size of the water extractable organic N (WEON) pool was similar in size to the inorganic N pool. In contrast, the KCl extractable organic N (KClEON) pool constituted the dominant form of soluble N in soils under all land uses, perhaps indicating that significant amounts were held on the soil exchange phase. In contrast to inorganic N, which varied significantly with land use, the size of the KClEON and WEON pool was similar for all land uses with the exception of KClEON in tilled land, where significantly lower amounts were observed. We conclude that SON constitutes an important soil N pool in a broad range of land uses, and that its role in microbial N assimilation, plant nutrition and ecosystem responses to atmospheric N deposition warrants further attention.

Dynamic simulation models are increasingly used in environmental and agricultural science. Here we present a method that allows models to be used to determine optimum timing of sampling for field trials. The model is used to decide when to concentrate sampling effort before the field trial begins. The model chosen to design sampling strategy should include an appropriately sensitive description of all processes that influence measurements significantly. The simulation is run, using predicted weather data, to generate the full time series before the trial begins. Every point in the simulation is considered initially to be a potential sampling point. The potential error due to not including a measurement at each point is calculated using the ‘dot-to-dot’ method of b10Smith et al. (2002) by omitting simulated values consecutively. The calculated potential error provides a measure of the priority that should be given to sampling at each point. Where the error introduced by omitting the simulated value exceeds an acceptable error, the value at the last discernible time step should be measured so that all statistically significant changes in the system can be observed. The output from the calculation is a plan of sampling times needed to capture all statistically significant events that are likely to occur over the course of the trial.

Dynamic simulation models are important tools for rationalizing complex changes in soil organic matter. Most such models for organic matter can be described as having either a pool structure or a continuous one. A pool structure can offer advantages in ease of use and transferability. Some pools are easily measured, whereas others cannot be measured directly. New methods of fractionation are being developed in an attempt to base models on measurable fractions. A requirement for such models is a demonstration that the measured fraction and model pool are equivalent. A measured fraction is equivalent to a model pool only if, within acceptable limits, it is unique as well as non-composite. If the measured fraction is not unique, describing it as a separate pool adds no extra information, while the added complexity can increase propagation of errors. If it is composite then the characteristics of the fraction will change with changing environment as a result of changing proportions of subpools. This will produce a model that cannot be applied without deriving parameters afresh: such a model is of greatly reduced value. Here we develop methods to examine if a fraction is both unique and non-composite. The tests for unique and composite pools were applied to the SUNDIAL (simulation of nitrogen dynamics in arable land) model of organic matter and nitrogen turnover in soil. Results suggest that the debris, biomass and humus pools are unique, but biomass and humus are composed of two or more subpools. This worked example illustrates how, given suitable data, any pool-based model can be tested by these methods.

The existence of soluble organic forms of N in rain and drainage waters has been known for many years, but these have not been generally regarded as significant pools of N in agricultural soils. We review the size and function of both soluble organic N extracted from soils (SON) and dissolved organic N present in soil solution and drainage waters (DON) in arable agricultural soils. SON is of the same order of magnitude as mineral N and of equal size in many cases; 20–30 kg SON-N ha–1 is present in a wide range of arable agricultural soils from England. Its dynamics are affected by mineralisation, immobilisation, leaching and plant uptake in the same way as those of mineral N, but its pool size is more constant than that of mineral N. DON can be sampled from soil solution using suction cups and collected in drainage waters. Significant amounts of DON are leached, but this comprises only about one-tenth of the SON extracted from the same soil. Leached DON may take with it nutrients, chelated or complexed metals and pesticides. SON/DON is clearly an important pool in N transformations and plant uptake, but there are still many gaps in our understanding.

Nitrate present in arable soils in autumn is at risk to leaching during the following winter. To see whether unused nitrogen fertiliser was a major source of this nitrate, 15N-labelled fertiliser was applied to 11 winter wheat crops at rates of between 47 and 234 kg N ha−1in spring. The experiments were on three contrasting soil types in south-east England. On average, 17′% of the N from spring-applied labelled fertiliser remained in the 0–23 cm soil layer at harvest (range, 7–36%) but only a small proportion was in inorganic forms (ammonium + nitrate). This was never more than 5 kg N ha−1and averaged only 1·3% of the fertiliser N applied (range, 0·4–3·6 %). Between 79 and 98% of the inorganic N in soils at harvest was unlabelled, being derived from the mineralisation of organic N rather than from unused fertiliser. The amount of unlabelled N was much greater where wheat was grown after ploughing up grass or grass/clover leys than where it was grown in all-arable rotations. When wheat was grown without N fertiliser, soil inorganic N content at harvest was no lower than in plots given fertiliser at rates up to 234 kg N ha−1. This work indicates that, for soil growing winter wheat, almost all of the nitrate at risk to leaching over the winter period comes from mineralisation of organic N, not from unused fertiliser applied in spring. Consequently, even a drastic reduction in N fertiliser use would have little effect on nitrate leaching.