The potential to increase grassland soil C stocks by extending reseeding intervals is dependent on soil texture and depth.

Grasslands account for ∼30% of global terrestrial carbon (C), of which most is stored in soils and provide important ecosystem services including livestock and forage production. Reseeding of temporary grasslands on a 5-year cycle is a common management practice to rejuvenate sward productivity and reduce soil compaction, but is physically disruptive and may reduce soil organic carbon (SOC) stocks. However, research to date is limited, which impacts on the ability to optimise grassland management for climate change mitigation. To determine whether extending the time interval up to 20 years between grassland reseeding can increase stable SOC stocks, a soil survey was conducted across three UK grassland chrono-sequences comprising 24 fields on contrasting soil types. We found that grassland SOC stocks (39.8-114.8 Mg C ha-1) were higher than co-located fields in arable rotations (29.3-83.2 Mg C ha-1) and the relationship with grassland age followed a curvilinear relationship with rapid SOC stock accumulation in the year following reseeding (2.69-18.3 Mg C ha-1 yr-1) followed by progressively slower SOC accumulation up to 20 years. Contrary to expectation, all grasslands had similar soil bulk densities and sward composition questioning the need for traditional 5-year reseeding cycles. Fractionation of soils into stable mineral associated fractions revealed that coarse textured grassland topsoils (0-15 cm) were near-saturated in C irrespective of grassland age whilst loam soils reached saturation ∼10 years after reseeding. Fine-textured topsoils and subsoils (15-30 cm) of all textures were under saturated and thus appear to hold the most potential to accrue additional stable C. However, the lack of a relationship between C saturation deficit and grassland age in subsoils suggests that more innovative management to promote SOC redistribution to depth, such as a switch to diverse leys or full inversion tillage may be required to maximise subsoil SOC stocks. Taken together our findings suggest that extending the time between grassland reseeding could temporarily increase SOC stocks without compromising sward composition or soil structure. However, detailed monitoring of the trade-offs with grassland productivity are required. Fine textured soils and subsoils (15-30 cm) have the greatest potential to accrue additional stable C due to under saturation of fine mineral pools.

Global environmental changes impact soil hydraulic functions through biophysical feedbacks.

Although only representing 0.05% of global freshwater, or 0.001% of all global water, soil water supports all terrestrial biological life. Soil moisture behaviour in most models is constrained by hydraulic parameters that do not change. Here we argue that biological feedbacks from plants, macro-fauna and the microbiome influence soil structure, and thus the soil hydraulic parameters and the soil water content signals we observe. Incorporating biological feedbacks into soil hydrological models is therefore important for understanding environmental change and its impacts on ecosystems. We anticipate that environmental change will accelerate and modify soil hydraulic function. Increasingly, we understand the vital role that soil moisture exerts on the carbon cycle and other environmental threats such as heatwaves, droughts and floods, wildfires, regional precipitation patterns, disease regulation and infrastructure stability, in addition to agricultural production. Biological feedbacks may result in changes to soil hydraulic function that could be irreversible, resulting in alternative stable states (ASS) of soil moisture. To explore this, we need models that consider all the major feedbacks between soil properties and soil-plant-faunal-microbial-atmospheric processes, which is something we currently do not have. Therefore, a new direction is required to incorporate a dynamic description of soil structure and hydraulic property evolution into soil-plant-atmosphere, or land surface, models that consider feedbacks from land use and climate drivers of change, so as to better model ecosystem dynamics.

Zones of influence for soil organic matter dynamics: A conceptual framework for data and models.

Soil organic matter (SOM) is an indicator of sustainable land management as stated in the global indicator framework of the United Nations Sustainable Development Goals (SDG Indicator 15.3.1). Improved forecasting of future changes in SOM is needed to support the development of more sustainable land management under a changing climate. Current models fail to reproduce historical trends in SOM both within and during transition between ecosystems. More realistic spatio-temporal SOM dynamics require inclusion of the recent paradigm shift from SOM recalcitrance as an 'intrinsic property' to SOM persistence as an 'ecosystem interaction'. We present a soil profile, or pedon-explicit, ecosystem-scale framework for data and models of SOM distribution and dynamics which can better represent land use transitions. Ecosystem-scale drivers are integrated with pedon-scale processes in two zones of influence. In the upper vegetation zone, SOM is affected primarily by plant inputs (above- and belowground), climate, microbial activity and physical aggregation and is prone to destabilization. In the lower mineral matrix zone, SOM inputs from the vegetation zone are controlled primarily by mineral phase and chemical interactions, resulting in more favourable conditions for SOM persistence. Vegetation zone boundary conditions vary spatially at landscape scales (vegetation cover) and temporally at decadal scales (climate). Mineral matrix zone boundary conditions vary spatially at landscape scales (geology, topography) but change only slowly. The thicknesses of the two zones and their transport connectivity are dynamic and affected by plant cover, land use practices, climate and feedbacks from current SOM stock in each layer. Using this framework, we identify several areas where greater knowledge is needed to advance the emerging paradigm of SOM dynamics-improved representation of plant-derived carbon inputs, contributions of soil biota to SOM storage and effect of dynamic soil structure on SOM storage-and how this can be combined with robust and efficient soil monitoring.

Co-located ecological data for exploring top- and subsoil carbon dynamics across grassland-woodland contrasts.

Soil organic carbon (SOC) is a soil health indicator and understanding dynamics changing SOC stocks will help achieving net zero goals. Here we present four datasets featuring 11,750 data points covering co-located aboveground and below-ground metrics for exploring ecosystem SOC dynamics. Five sites across England with an established land use contrast, grassland and woodland next to each other, were rigorously sampled for aboveground (n = 109), surface (n = 33 soil water release curves), topsoil, and subsoil metrics. Commonly measured soil metrics were analysed in five soil increments for 0-1 metre (n = 4550). Less commonly measured soil metrics which were assumed to change across the soil profile were measured on a subset of samples only (n = 3762). Additionally, we developed a simple method for soil organic matter fractionation using density fractionation which is part of the less common metrics. Finally, soil metrics which may impact SOC dynamics, but with less confidence as to their importance across the soil profile were only measured on topsoil (~5-15 cm = mineral soil) and subsoil (below 50 cm) samples (n = 2567).

Plant and soil communities are associated with the response of soil water repellency to environmental stress.

A warming climate and expected changes in average and extreme rainfall emphasise the importance of understanding how the land surface routes and stores surface water. The availability and movement of water within an ecosystem is a fundamental control on biological and geophysical activity, and influences many climatic feedbacks. A key phenomenon influencing water infiltration into the land surface is soil hydrophobicity, or water repellency. Despite repellency dictating the speed, volume and pattern of water infiltration, there is still major uncertainty over whether this critical hydrological process is biologically or physicochemically controlled. Here we show that soil water repellency is likely driven by changes in the plant and soil microbial communities in response to environmental stressors. We carried out a field survey in the summers of 2013 to 2016 in a variety of temperate habitats ranging across arable, grassland, forest and bog sites. We found that moderate to extreme repellency occurs in 68% of soils at a national scale in temperate ecosystems, with 92% showing some repellency. Taking a systems approach, we show that a wetter climate and low nutrient availability alter plant, bacterial and fungal community structure, which in turn are associated with increased soil water repellency across a large-scale gradient of soil, vegetation and land-use. The stress tolerance of the plant community and associated changes in soil microbial communities were more closely linked to changes in repellency than soil physicochemical properties. Our results indicate that there are consistent responses to diverse ecosystem stresses that will impact plant and microbial community composition, soil properties, and hydrological behaviour. We suggest that the ability of a biological community to induce such hydrological responses will influence the resilience of the whole ecosystem to environmental stress. This highlights the crucial role of above-belowground interactions in mediating climatic feedbacks and dictating ecosystem health.

Divergent national-scale trends of microbial and animal biodiversity revealed across diverse temperate soil ecosystems.

Soil biota accounts for ~25% of global biodiversity and is vital to nutrient cycling and primary production. There is growing momentum to study total belowground biodiversity across large ecological scales to understand how habitat and soil properties shape belowground communities. Microbial and animal components of belowground communities follow divergent responses to soil properties and land use intensification; however, it is unclear whether this extends across heterogeneous ecosystems. Here, a national-scale metabarcoding analysis of 436 locations across 7 different temperate ecosystems shows that belowground animal and microbial (bacteria, archaea, fungi, and protists) richness follow divergent trends, whereas ß-diversity does not. Animal richness is governed by intensive land use and unaffected by soil properties, while microbial richness was driven by environmental properties across land uses. Our findings demonstrate that established divergent patterns of belowground microbial and animal diversity are consistent across heterogeneous land uses and are detectable using a standardised metabarcoding approach.

Experimental evidence for drought induced alternative stable states of soil moisture.

Ecosystems may exhibit alternative stable states (ASS) in response to environmental change. Modelling and observational data broadly support the theory of ASS, however evidence from manipulation experiments supporting this theory is limited. Here, we provide long-term manipulation and observation data supporting the existence of drought induced alternative stable soil moisture states (irreversible soil wetting) in upland Atlantic heath, dominated by Calluna vulgaris (L.) Hull. Manipulated repeated moderate summer drought, and intense natural summer drought both lowered resilience resulting in shifts in soil moisture dynamics. The repeated moderate summer drought decreased winter soil moisture retention by ~10%. However, intense summer drought, superimposed on the experiment, that began in 2003 and peaked in 2005 caused an unexpected erosion of resilience and a shift to an ASS; both for the experimental drought manipulation and control plots, impairing the soil from rewetting in winter. Measurements outside plots, with vegetation removal, showed no evidence of moisture shifts. Further independent evidence supports our findings from historical soil moisture monitoring at a long-term upland hydrological observatory. The results herald the need for a new paradigm regarding our understanding of soil structure, hydraulics and climate interaction.

Spatial patterns and environmental constraints on ecosystem services at a catchment scale.

Improved understanding and prediction of the fundamental environmental controls on ecosystem service supply across the landscape will help to inform decisions made by policy makers and land-water managers. To evaluate this issue for a local catchment case study, we explored metrics and spatial patterns of service supply for water quality regulation, agriculture production, carbon storage, and biodiversity for the Macronutrient Conwy catchment. Methods included using ecosystem models such as LUCI and JULES, integration of national scale field survey datasets, earth observation products and plant trait databases, to produce finely resolved maps of species richness and primary production. Analyses were done with both 1×1km gridded and subcatchment data. A common single gradient characterised catchment scale ecosystem services supply with agricultural production and carbon storage at opposing ends of the gradient as reported for a national-scale assessment. Species diversity was positively related to production due to the below national average productivity levels in the Conwy combined with the unimodal relationship between biodiversity and productivity at the national scale. In contrast to the national scale assessment, a strong reduction in water quality as production increased was observed in these low productive systems. Various soil variables were tested for their predictive power of ecosystem service supply. Soil carbon, nitrogen, their ratio and soil pH all had double the power of rainfall and altitude, each explaining around 45% of variation but soil pH is proposed as a potential metric for ecosystem service supply potential as it is a simple and practical metric which can be carried out in the field with crowd-sourcing technologies now available. The study emphasises the importance of considering multiple ecosystem services together due to the complexity of covariation at local and national scales, and the benefits of exploiting a wide range of metrics for each service to enhance data robustness.

Investigations of freezing and cold storage for the analysis of peatland dissolved organic carbon (DOC) and absorbance properties.

Although measured rates of biological degradation of DOC are typically low under dark conditions, it is assumed that water samples must be analysed soon after collection to provide an accurate measure of DOC concentration and UV-visible absorbance. To examine the impact of storage on DOC quality and quantity, we took water samples from an ombrotrophic peatland, and stored them in the dark at 4 °C for 138-1082 days. A median of 29% of DOC was lost during storage, but losses of absorbance at 254 nm were less. DOC loss followed a first-order exponential decay function, and was dependent on storage time. DOC half-life was calculated as 1253 days. Specific absorbance at 254 nm suggested that samples containing more aromatic DOC were more resistant to degradation, although time functioned as the primary control. Samples from two fens showed that loss of absorbance was greater at 400 nm rather than 254 nm, after 192 days storage, suggesting that non-aromatic DOC is preferentially degraded. These results suggest that samples can be stored for several months before losses of DOC become detectable, and that it is possible to back-calculate initial DOC concentrations in long-term stored samples based on known decay rates. Freeze/thaw experiments using samples from a range of peatlands suggested that DOC concentration was mostly unaffected by the process, but DOC increased 37% in one sample. Freezing had unpredictable and sometimes strong effects on absorbance, SUVA and E ratios, therefore freezing is not recommended as a method of preservation for these analyses.

UV-visible absorbance spectroscopy as a proxy for peatland dissolved organic carbon (DOC) quantity and quality: considerations on wavelength and absorbance degradation.

Absorbance in the UV or visible spectrum (UV-vis) is commonly used as a proxy for DOC concentrations in waters draining upland catchments. To determine the appropriateness of different UV-vis measurements we used surface and pore water samples from two Welsh peatlands in four different experiments: (i) an assessment of single wavelength proxies (1 nm increments between 230-800 nm) for DOC concentration demonstrated that 254 nm was more accurate than 400 nm. The highest R(2) values between absorbance and DOC concentration were generated using 263 nm for one sample set (R(2) = 0.91), and 230 nm for the other three sample sets (respective R(2) values of 0.86, 0.81, and 0.93). (ii) A comparison of different DOC concentration proxies, including single wavelength proxies, a two wavelength model, a proxy using phenolic concentration, and a proxy using the area under a UV spectrum at 250-350 nm. It was found that both a single wavelength proxy (≤263 nm) and a two wavelength model performed well for both pore water and surface water. (iii) An evaluation of the E2 : E3, E2 : E4, E4 : E6 ratios, and SUVA (absorbance at 254 nm normalised to DOC concentration) as indicators of DOC quality showed that the E4 : E6 ratio was subject to extensive variation over time, and was highly correlated between surface water and pore water, suggesting that it is a useful metric to determine temporal changes in DOC quality. (iv) A repeated weekly analysis over twelve weeks showed no consistent change in UV-vis absorbance, and therefore an inferred lack of degradation of total DOC in samples that were filtered and stored in the dark at 4 °C.