Soil quality and ecosystem services of peat grasslands

Samenvatting

Worldwide, drained peat covers only 0.3% of the land area but net decomposition of the organic matter under aerobic conditions contributes 6% to anthropogenic CO2 emissions. For drained peat grasslands in the Netherlands, society requires from farmers and other land owners not only that peat decomposition is reduced, but also that key elements of biodiversity such as meadow birds are supported, that soils can buffer extreme weather events, and that nutrient losses are minimized. Thus, agriculture is no longer required to deliver provisioning services (food production) only, but increasingly also supporting and regulating ecosystem services.

The overall aim of this thesis is to get insight in soil quality of peat grasslands in relation to the ecosystem services support of biodiversity, climate regulation, water regulation and grass production, and to evaluate the effects of land use and land management from this integral perspective. To this end, soil and vegetation measurements were carried out in experiments in dairy and semi-natural grasslands across the western peat region of the Netherlands.

The effects of land use on the delivery of ecosystem services was assessed by comparing twenty dairy and twenty semi-natural grasslands, differing in drainage depth, grazing, mowing and fertilization intensity. Chapter 2 presents botanical parameters and a comprehensive set of biotic and abiotic parameters from the topsoil of these grasslands. The dairy grasslands had a drier, nutrient richer and more productive topsoil, a more abundant soil fauna and higher water infiltration capacity than the semi-natural grasslands. The semi-natural grasslands were lower in floral and soil α-diversity, but higher in γ-diversity. This could be explained from differences in resource availability and spatial heterogeneity between the land uses. Potential C mineralization rates were similar in dairy and semi-natural grasslands. However, C mineralization was based on a bacterial decomposition pathway and was limited by drought in the dairy grasslands, whereas it was based on a more fungal decomposition pathway in semi-natural grasslands.

Due to large variation in unfertilized grass N yield (a proxy for soil N supply) across dairy grasslands on peat, the current fertilization guideline based on a fixed soil N supply may induce unnecessary large N losses. The aim of Chapter 3 is to develop a prediction tool for soil N supply of dairy grasslands on peat and ultimately help reduce losses. No match was found between the variation in soil N supply and that in soil N mineralization as estimated with soil ecological data. However, the soil parameters Ca:Mg ratio and C to soil organic matter (C:SOM) ratio were statistically the best predictors of soil N supply and fertilized grass yield, respectively. Those parameters were related to soil structure and water availability. It was concluded that soil parameters predicted part of the spatial variation in grass yield with indicators of soil physical-hydrological properties, without direct link with N mineralization.

Based on the results of Chapter 3, a factorial field experiment on three peat grasslands was carried out in with the soil Ca:Mg ratio was changed by adding two minerals that influenced pH (CaCO3 and MgCO3) and two (CaSO4 and MgSO4) that did not (Chapter 4). Within the two years of the experiment, the unfertilized grass N yield was influenced by changes in soil pH and organic matter mineralization mainly in the first year, but not by the Ca:Mg ratio. The addition of lime (CaCO3) increased the soil pH and reduced total N and C, indicating significant soil N and C mineralization. From the different treatment effects on pH, SOM and hot water-extractable C (HWC, a labile C pool), it was deduced that the pH-related SOM decomposition depends also on P availability and Ca binding. Therefore, it was concluded that manipulating the soil Ca:Mg ratio does not influence grass N uptake in the first two years after application of the minerals. Moreover, to avoid soil losses of C and N, the current agricultural advice of pH management in peat grasslands should be better adapted to local soil properties.

In another field experiment (Chapter 5), the application of organic and inorganic fertilizers during three consecutive growing seasons was evaluated as a measure to combine ecosystem services of peat grasslands. Fertilizers were applied at a fixed, moderate amount of total N (120 kg ha−1 yr−1) that resulted in different amounts of total C per treatment due to differences in fertilizer C:N ratios. An increase in detritivorous (epigeic) earthworms was found in plots to which the solid fraction of slurry manure had been added, which was the fertilizer with the highest C:N ratio and input of organic matter. A similar effect was observed where sawdust was combined with N fertilizer. Compared to the fertilizers with lower C:N ratios, those treatments also promoted arbuscular mycorrhizal fungi, which possibly explains the lack of negative effect on grass yield. It was concluded that the use of organic fertilizers with high and non-humified organic matter content can be part of a regeneration strategy of ecosystem services in peat grasslands.

This thesis provides an integral insight on the delivery of ecosystem services in peat grasslands, and explores ways to use and manage peat soils to address the demands of society. In the general discussion (Chapter 6), it is concluded that soil biotic and abiotic quality forms the basis of vital ecosystem services delivered by peat grasslands, and that managing those sometimes conflicting ecosystem services is inextricably related to influencing the dynamics of soil organic matter and soil hydrology. To face the challenges of agriculture on peatland in the coming decades, a fertile cooperation of farmers with nature conservation organizations and the society as a whole will be needed.