Do UK grasslands have the ability to sequester more carbon? Assessment of stability and resilience to changing climate and management

Abstract

Soil organic matter (SOM) is the largest terrestrial pool of organic carbon (C). Carbon can persist in the soil due to several mechanisms, i) inherent chemical recalcitrance, ii) association with minerals in the soil and iii) limitations to microbial decomposition due to inaccessibility within aggregates, or conditions which limit microbial decomposition. However, C within mineral associated organic matter (MAOM-C), is often considered to be the most persistent, and consequently efforts to increase SOC largely focus on this long-lived SOC pool. The careful management of agricultural soils can contribute to climate mitigation by increasing C within MAOM, with co-benefits for ecosystem services and contributes to food security by increasing soil fertility. However, uncertainty remains with respect to the maximum sequestration potential, and the response of MAOM-C to changes in climate and management. This thesis aims to determine the carbon sequestration potential of agricultural grasslands, and to examine the effect of climate and management on MAOM-C formation within the United Kingdom.

Estimating the C sequestration potential of soils is essential in determining their potential contribution to climate mitigation policies. However, the myriad of processes that contribute to the formation, and persistence of soil C makes this difficult. Consequently, a balance must be struck between the accuracy of estimates and resource inputs. In this thesis a relatively simple, but widely used linear regression equation, developed by Hassink (1997), to estimate maximum C was examined in conjunction with boundary line and quantile regression analysis – both suggested as options to overcome the shortcomings of linear regression. The quantile regression estimate of maximum soil organic C was almost double that of the linear regression and boundary line analysis (0.89 ± 0.074, 0.43 ± 0.017 and 0.57 ± 0.052 g C per kg soil, respectively). Additionally, the linear regression generated from the selected UK grasslands, was significantly different to that of Hassink (1997), demonstrating the importance of estimation methods which account for in situ context, such as management and climatic factors which play a role in C accrual.

To date, much of the focus on stability and resilience of persistent soil C has focused on its mineralisation and losses due to climatic and management changes. However relatively less focus has been paid to the influence of these factors on the formation of persistent soil C. This thesis examines the influence of management factors relevant to agricultural grasslands, and climatic change on the formation of MAOM-C. Nitrogen fertilisers are frequently used to enhance plant productivity, increasing plant C inputs to the soil. Additionally, nitrogen is thought to play an important role in the formation of organo-mineral associations. Therefore, the effect of the addition of ammonium nitrate and the quantity of C substrate on MAOM-C formation was examined. It was anticipated that greater C inputs would result in greater absolute retention of substrate within MAOM-C, but less proportionally, due to changes in microbial C use efficiency (the amount incorporated into biomass versus respired as carbon dioxide). It was found that absolute substrate C retention increased with higher C addition rates, with no apparent effect on proportional retention within MAOM-C. The results also suggest that the role of nitrogen in the retention of labile substrate C, such as glucose, may be a function of C addition rate.

Looking to the future it is necessary to understand how warming will influence the formation of MAOM-C. This was examined in conjunction with the effect of substrate type, to improve the understanding of the effects of substrate type and temperature on MAOM-C formation.

There was no effect of temperature or substrate type (glucose versus acetic acid) on substrate C recovered in MAOM-C. However, substrate C recovery was significantly affected by substrate and temperature in the bulk SOC. The substrate C recovered in bulk SOC was significantly higher for glucose than acetic acid at 10 and 15°C, but not at 20 and 25°C. Suggesting that MAOM-C formation was independent of substrate type and temperature, but that substrate type and temperature have an influence on retention of fresh labile C inputs within the bulk soil, possibly within dissolved organic C or microbial biomass. An important aspect of agricultural land management is the maintenance of soil pH. Changes to soil pH can influence the retention of C within the soil by altering the bonds between organic matter and minerals in the soil. In acidic conditions C may be lost due to disruption of organo-mineral associations and reduced microbial activity. Whilst, in alkaline conditions additional C may be preserved due to cation bridging. Soils from a long-term pH trial were used to examine the effect of soil pH, grass ley duration and depth on MAOM-C. Ley duration had no effect on total SOC or MAOM-C, and as expected both declined with depth. In the topsoil, both SOC and MAOM-C increased with soil pH, possibly due to additional abiotic cation stabilisation, due to the presence of calcium, magnesium and manganese associated with lime application in pH management.

The findings of this thesis contribute to the understanding of the current C status of a selection of UK grasslands, and how typical management and climatic factors influence the formation and persistence of MAOM-C. There is likely to be potential for additional C sequestration within persistent C pools within agricultural grasslands in the UK. However, further work is required to improve the understanding of the formation and persistence of MAOM-C in response to typical land managements. This would help to guide land management policies which enhance and limit losses of existing SOC.