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While rhizosphere priming effects are well-documented under laboratory and controlled-environment conditions, their significance in undisturbed systems under field conditions is less clear. This is in part because it is impracticable to measure rates of rhizodeposition in the field with high resolution over a substantial period. We propose that photosynthesis, closely linked to rhizodeposition, can be used as a proxy for plant root activity.We have used a field system containing 24 0.8-m diameter, 1-m deep lysimeters holding naturally-structured soil monoliths from two contrasting C3 soils sown with a C4 grass (Bouteloua dactyloides) to measure carbon (C) fluxes at a high temporal resolution, exploiting isotopic differences to allow partitioning of plant and soil fluxes. These fluxes are coupled to high-resolution measurements of soil temperature and moisture, alongside atmospheric temperature and solar radiation. This system has allowed measurement of both ecosystem resolution and net ecosystem exchange, and the partitioning of respiration between plant and soil sources using stable isotope methods. Using this system we have generated a dataset of measured and modelled respiration and photosynthesis fluxes over two years.Our dataset has revealed clear seasonal and diurnal patterns in plant and soil fluxes. We have assessed the relationship between diurnal patterns in soil respiration and potential drivers, and examined whether model estimates of soil respiration are improved by the inclusion of photosynthesis as an explanatory variable alongside soil moisture and temperature. We found a significant positive relationship between photosynthesis and soil respiration, and inclusion of photosynthesis improves models of soil respiration. This is best explained by rhizosphere priming enhancing soil C turnover.

Our understanding of soil carbon (C) dynamics is limited; field measurements necessarily conflate fluxes from plant and soil sources and we therefore lack long-term field-scale data on soil C fluxes to use to test and improve soil C models. Furthermore, it is often unclear whether findings from lab-based studies, such as the presence of rhizosphere priming, apply to soil systems in the field. It is particularly important that we are able to understand the roles of soil temperature and moisture, and plant C inputs, as drivers of soil C dynamics in order to predict how changing climate and plant productivity may affect the net C balance of soils. We have developed a field laboratory with which to generate much-needed long-term C flux data under field conditions, giving near-continuous measurements of plant and soil C fluxes and their drivers.The laboratory contains 24 0.8-m diameter, 1-m deep, naturally-structured soil monoliths of two contrasting C3 soils (a clay-loam and a sandy soil) in lysimeters. These are sown with a C4 grass (Bouteloua dactyloides), providing a large difference in C isotope signature between C4 plant respiration and C3-origin soil organic matter (SOM) decomposition, which enables clear partitioning of the net C flux. This species is used as a pasture grass in the United States, and regular trimming through the growing season simulates low-intensity grazing. The soil monoliths are fitted with gas flux chambers and connected via an automated sampling loop to a cavity ring-down spectrometer, which measures the concentration and 12C:13C isotopic ratio of CO2 during flux chamber closure. Depth-resolved measurements of soil temperature and moisture in each monolith are made near-continuously, along with measurements of incoming solar radiation, rainfall, and air temperature a the field site. The gas flux chambers are fitted with removable reflective backout covers allowing flux measurements both incorporating, and in the absence of, photosynthesis.We have collected net ecosystem respiration data, measurements of photosynthesis, and recorded potential drivers of respiration over two growing seasons through 2018 and 2019. Through partitioning fluxes between plant respiration and SOM mineralisation we have revealed clear diurnal trends in both plant and soil C fluxes, along with overarching seasonal trends which modify both the magnitude of fluxes and their diurnal patterns. Rates of photosynthesis have been interpolated between measurement periods using machine learning to generate a predictive model, which has allowed us to investigate the effect of plant productivity on SOM mineralisation and assess whether rhizosphere priming can be detected in our system. Through regression analyses and linear mixed effects modelling we have evaluated the roles of soil temperature, soil moisture, and soil N content as drivers of variation in plant and soil respiration in our two contrasting soils. This has shown soil temperature to be the most important control on SOM mineralisation, with soil moisture content playing only a minor role. We have also used our empirical models to suggest how the carbon balance of pasture and grassland soils may respond to warming temperatures.

Soil organic matter (SOM) is the largest store of organic carbon (C) in the biosphere, but still the turnover of SOM is incompletely understood and not well described in global C cycle models. Here we use the Community Land Model (CLM) and compare the output for soil organic C (SOC) to estimates from a global data set. We also modify the assumptions about SOM turnover in two ways: (1) we assume distinct temperature sensitivities of SOC pools with different turnover time and (2) we assume a priming effect, such that decomposition rate of native SOM increases in response to a supply of fresh organic matter. The standard model predicted the global distribution of SOM reasonably well in most areas, but it failed to predict the very high stocks of SOM at high latitudes. It also predicted somewhat too much SOC in areas with high plant productivity, such as tropical rain forests and some mid-latitude areas. Assuming that the temperature sensitivity of SOC decomposition is dependent on the turnover rate of component pools reduced total SOC at equilibrium by a relatively small amount (