Effects of grassland management intensification on dynamics of soil organic carbon and nitrogen in temperate grassland soils

In Germany, about one third of the agricultural area is managed as permanent grassland. The predominant use of permanent grassland in Germany is mostly the production of fodder for meat or milk production, achieved by grazing or mowing. In addition, an increased demand for biomass to produce renewable energy has influenced grassland management in the last decade. Until 2013, a continuous decrease in grassland due to grassland conversion was recorded (from 2003 to 2013, by a total of about 5 %). The reduction in permanent grassland was accompanied by an increase in intensification of grassland in many places. Therefore, the intensification of grassland management is typically conducted via higher grazing pressure or increased average use by mowing, combined with the application of fertilizers or an increase in fertilizer delivery. However, especially for farming in naturally small-scale, mountainous landscapes with less productive soils, extensive grazing is recommended for coincidently achieving meat and milk production as well as biodiversity goals. Also of interest is the renewal of grassland that is entirely or partially carried out, especially in intensively-used grassland, to improve efficiency. Grassland renewal can be performed with or without subsequent tillage. It is generally known that management intensification of grassland, grassland renewal, or conversion to arable land influences carbon and nitrogen stocks and dynamics in soil. From these current circumstances, the following objectives have been deduced for my doctoral thesis: (I) to evaluate the impact of different extensive grazing pressures on carbon and nitrogen in soil, on the coupling of carbon and nitrogen in soil, on soil microbial carbon, on the basal respiration of soil, and on mineral nitrogen in soil. (II) to evaluate the impact of varying, frequent mowing in combination with and without mineral N fertilization on carbon and nitrogen in soil, on the coupling of carbon and nitrogen in soil, on soil microbial carbon, on ergosterol as a marker of fungal biomass, on soil aggregate size class distribution and its carbon contents, and on the composition of soil organic matter. 7 Summary (III) to investigate the temporal dynamics during grassland renewal from the chemical destruction of the existing vegetation and reseeding either by direct seeding or by prior tillage, and to investigate their effects on organic carbon and nitrogen in soil, on soil microbial carbon, on soil aggregate size class distribution, and on their carbon contents. For the processing of (I), soil samples were taken during a long-term grazing experiment northwest of Göttingen (FOR BIOdiversity BENefit trial in Solling, Relliehausen), from three different extensively-grazed treatments in three soil depths (0-10 cm, 10-25 cm, and 25-40 cm) in April 2013. The different grazing pressures were determined by measuring the compressed pasture height during the vegetation period, using a rising-plate meter and subsequently adjusting the stocking rate. The soil samples were analyzed for soil organic carbon (SOC) and nitrogen (Nt) contents, the coupling of carbon and nitrogen, soil microbial carbon (Cmic), basal respiration, and mineral nitrogen (Nmin). Furthermore, pH, clay content, oxalate soluble aluminum, and iron contents were determined. Before data analysis, weighted means were calculated for the stocks of SOC, Nt, Nmin, and Cmic concentrations and basal respiration rates with the proportion of each compressed pasture height class as a weighting factor. A two-factorial analysis of variance (ANOVA) with the factor grazing intensity and the factor block was used in case of normality of residuals and homoscedasticity; otherwise, a Welch ANOVA was used. The weighted stocks of SOC, Nt, Cmic, and basal respiration were not significantly affected (p ≤ 0.05) by grazing intensity. The data were highly heterogeneous, which was probably caused by the heterogeneous soil mineralogy as well as by uncertainties in the analytical determination of the respective contents, the determination of bulk densities, and the stone contents. Thus, a large part of the variation of the organic carbon and total nitrogen contents can be explained regarding the content of oxalate soluble iron (multiple linear regression: R2 = 0.64). However, the contents of microbial carbon (R2 = 0.96) and the basal respiration rate (R2 = 0.9) are, in turn, explained to a significant extent by the organic carbon contents. Furthermore the possible effects of grazing intensity on the SOC and Nt stocks are presumably explained by the mineralogical variability. However, the low variability of the C/N ratios of different grazing intensities is attributed to a coupling of C and N and, in turn, suggests sufficient SOC and Nt supply in the extensive FORBIOBEN grazing trial. 8 Summary For the processing of (II), soil samples were taken in three soil depths up to 60 cm in depth in a grassland trial near Kiel managed by the Christian Albrechts University of Kiel. The grassland was established uniformly in 2004 after its preceding management as arable land. The trial was established in a randomized block design with three replicates and included these treatments: three cuts (3C) and five cuts (5C) per year; with and without N fertilization (N fertilization: 360 kg N ha-1 year-1 as calcium ammonium nitrate). The soil samples were analyzed for SOC and Nt stocks, Cmic and ergosterol as a marker for fungal biomass, soil aggregate size class distribution, their carbon contents, and the composition of soil organic matter (SOM). Three factorial ANOVAs with the factors cut (levels: 3 cuts and 5 cuts), fertilization (levels: N fertilization and no N fertilization), interaction between cut and fertilization, and factor block (three replicates organized in blocks) were conducted. The SOC stocks from the 5C regime compared to the 3C regimes in soil depths of 0 ~ 10 cm were significantly higher. This was presumably caused by higher harvesting frequencies, which promote the growth of tiller and leaf, are high in photosynthesis, and can stimulate biomass production. Additionally, the SOC stocks were significantly higher in the treatment without N fertilization in a soil depth of 0 ~ 10 cm. N fertilization may result in a decrease of SOC stocks by increasing microbial activity and altering the C substrate utilization pattern through changes in plant biomass composition. Furthermore, higher biomass yields under the 3C compared to the 5C regime as well as different plant species compositions in the treatments may have also contributed to SOM dynamics. Plant species composition was mainly influenced by the differing cut and fertilization regime. The main plant species under the 5C regime without N fertilization at the time of soil sampling were Lolium perenne and Trifolium repens (a highly productive grass and a legume, respectively). A dense root growth in Lolium perenne in a soil depth of 0 ~ 10 cm and the positive effects of legumes on SOC sequestration presumably caused the increase of the SOC stock under the 5C regime without N fertilization. The Nt stocks were significantly higher under the 5C regime in a depth of 0 ~ 10 cm in comparison to the 3C regime, which is presumably related to the occurrence of Lolium perenne under the 5C regime. Soil microbial C contents were significantly higher under the 5C regime than under the 3C regime, whereas N fertilization in significantly lower Cmic contents resulted in topsoil. This is presumably related to stimulated root exudation due to a higher harvesting frequency and complex changes in microbial competition and community structure due to the addition of N. Ergosterol contents in the surface soil were significantly higher under the 5C regime in comparison with the 3C regime, presumably caused by different root 9 Summary growth, exudation, and substrate quality. Cmic and the ergosterol contents were closely correlated to SOC stocks (Cmic: Spearman Rs = 0.81; ergosterol: Spearman Rs = 0.87) in topsoil. This indicates that, in the treatments that most likely resulted in higher root productions due to stimulation and plant species composition, microbial and fungal biomasses were stimulated. For the aggregate distribution in the topsoil layer, a shift from small (250-1000 μm) to large (> 2000 μm) macroaggregates from the 3C to the 5C regime was detected, presumably caused by a higher concentration of roots and root exudates related to Lolium perenne, with its high amount of fine roots under the 5C treatment in a soil depth of 0-10 cm. Five cuts per year affects SOC and Nt stocks positively, as well as contents of large macro-aggregates, Cmic, and ergosterol, which indicate positive effects on the soil fertility of 5C in comparison with 3C. The N fertilization resulted in slightly negative effects on SOC stocks and Cmic contents. However, as the plant species composition was strongly influenced by cut and fertilization, it is not possible to assign the results found to direct effects (e.g., stimulation of biomass production and root exudation) or to indirect effects due to a different plant species composition. For the processing of (III) a grassland trial was established on a continuous cut grassland located in Oldenburg in 2013. The trial was managed and supervised by the Chamber of Agriculture of Lower Saxony and the Thünen Institute. In June 2013, a field trial was established and the treatments were arranged in a randomized complete block design with three replicates and consist of • Treatment (i), chem: chemical sward killing with glyphosate, followed by direct seeding of grassland in 1-cm depth; • Treatment (ii), phys: chemical sward killing with glyphosate followed by the use of a rotary cultivator, a moldboard plow (25-cm deep), and a land packer, afterwards seeding of grassland; • Treatment (iii), continuous cut grassland as control. Soil samples were taken five times during August 2013 to October 2014, each replicated four times in three soil depths. SOC stocks, aggregate size classes, and organic carbon stored in aggregate size classes were analyzed using two-way analysis of variance 10 Summary with the factors grassland renovation (control, chem, and phys) and block (blocks 1 to 3). For further data analysis, Spearman ́s rank correlation and regression analysis was conducted. Soil carbon stocks showed no significant differences between the grassland renewal with or without plowing and control. This suggests that a grassland renovation, after neither a purely chemical destruction of sward nor a grassland renovation using a plow, had a direct impact on the loss of carbon stocks in the grassland soils. This is perhaps related to the rather low pH in the soil (on average, 4.8), resulting in hampered microbial biomass, which, in turn, hampered the degradation of organic material. However, grassland renewal after plowing leads to an increase in microaggregate concentrations six days after plowing, compared to the grassland control in the surface soil and in the soil profile. This indicates that one-time plowing had a direct impact on soil aggregates and destroyed macroaggregates, which fragmented into microaggregates. However, significant seasonal variations of macro- and microaggregate concentrations over time were visible. Higher macroaggregate concentrations were found during periods with increased rainfall following higher gravimetric water content in the soil, compared to periods with less rainfall and less gravimetric water content. Correlation analysis and multiple linear regression analysis have shown that the gravimetric soil water content had a putatively major influence on the distribution of aggregate size classes. Thus, lower gravimetric soil water content led to a higher concentration of microaggregates and a lower concentration of macroaggregates, while higher gravimetric soil water content led to lower microaggregate and higher macroaggregate concentrations. The physical renovation two and seven months after plowing also led to a significantly higher microaggregate concentration in the surface soil compared to the grassland renewal without plowing, while the concentrations in the permanent grassland were in between. In the chemical renovation, the larger roots of the dead plants might still be largely undecomposed, protecting macroaggregates from degeneration. In the physical renovation, plowing affects macroaggregates negatively by dismantling them into microaggregates via mechanical forces. Furthermore, dead roots were rearranged and breached, which caused a loss of stability in the soil matrix. However, a year after plowing and grassland renovation, no significant difference existed compared to the other treatments. In the beginning, no effect of grassland renovation on SOC in aggregates was visible. However, later microaggregates become more important than macroaggregates in the sequestration of SOC compared to the unplowed control. Because of the late response in SOC in 11 Summary aggregates after the plowing event, it seems that the indirect effects (for instance, the degradation of the root systems of the dead plants) on soil aggregates had a wider influence than the direct physical impact of the plow. The plowing event in the physical renovation caused direct physical destruction of the aggregates and, indirectly, destruction of the root systems of the dead plants. However, impacts on soil macroaggregates were nullified one year after grassland renovation. Chemical renovation resulted in higher macroaggregate concentrations compared with the physical renovation, especially two-to-seven months after the renovation, and in similar concentrations compared with the permanent grassland in any sampling time within one year. Presumably, dead plant roots could act as binding agents and stabilize aggregates for some time. The high temporal variation in the aggregate distribution within one year in the renovated as well as in the permanent grassland indicates that the soil moisture had a wide influence and that dry conditions in the soil led to a breakdown of larger aggregates. From the findings obtained above, it can be concluded that the impact of the intensified use of grassland on carbon and nitrogen dynamics is highly dependent on location. First, in the implementation and the degree of intensification, major differences have been observed. Thus, a direct comparison are not directly achievable. However, it can be stated that intensified grassland management, except with a high application rate of mineral N fertilizer and a large removal of biomass (with a mean of 1459 g m-2), does not or only has a low tendency to have negative impacts on the SOC and Nt stocks and sequestration mechanisms. Unlike the loss of biodiversity through intensified use, the SOC stocks and sequestration mechanisms, which are inevitably linked to plant species diversity and the variety of soil organisms, tend to react more slowly to management changes, while showing relatively rapid regeneration. Thus, targeted and appropriate management seems to be important when favoring SOC stocks and sequestration mechanisms.