- Gross N turnover and soil solution chemistry as influenced by fluctuations of soil water potential and water table in a Podzol and a fen soil (2011)
- Given the climate scenarios, the higher frequency of drying/rewetting cycles of soils in the future can be expected. These changes of the meteorological conditions likely result in an increasing frequent and intensive drought periods in summer, causing irregular and extreme drought stress in forest soils or a drawdown of water table in wetland ecosystems, which may influence the turnover of nutrients in soils to a larger extend than previously thought. The question arises how these climate changes will influence N and C turnover in forest and fen soils. A growing number of laboratory studies on drying/rewetting of soils have been published during past decades, but many studies used either disturbed soil samples or intact soil cores in laboratory. Although soil drying is a frequent phenomenon in the field, the long-term effects of drying/rewetting and irrigation on in situ fluxes and concentrations of solutes in forest and fen soils are unclear. Several studies have investigated the influence of soil water content on net N turnover rather than gross rates. Net ammonification and nitrification include two major processes: gross ammonification and gross nitrification on the one side and microbial immobilization on the other side. To identify the response of specific processes to soil drying, gross rates need to be measured. This thesis focused on the impact of changing water potential or water table level on gross N turnover rates and soil solution chemistry in two different ecosystems in South-Eastern Germany. In a Norway spruce forest, the effects of decreasing water potential and prolonged periods of summer drought on soil gross N turnover were investigated by laboratory and field experiments. Soil solutions and throughfall were collected and the cumulative in situ fluxes of DIN, DON and DOC with forest floor percolates were calculated. In a minerotrophic fen, we studied the response of N and C mineralization and soil solution chemistry to water table fluctuations in a laboratory experiment. In the field, we collected the soil pore water in 3 depths to clarify the long-term effects of water table level on the concentrations of solutes. Homogenized soil samples of the Oi+Oe, Oa and EA horizons were taken and adjusted to 6 different water potentials in the laboratory. In the field experiment, throughfall exclusion and irrigation plots were established to simulate different precipitation patterns of a dry and wet growing season. Gross N turnover rates were determined in undisturbed soil cores from Oi+Oe and Oa+EA horizons during the drying period and after rewetting. Soil drying decreased gross ammonification rates in the O horizon. The lowest rates were found at the throughfall exclusion plots but the differences to the irrigation and control plots were not statistically significant. A substantial ammonification rate of 14 mg N kg-1 soil day-1 was observed at 3.2 MPa (pF 4.5). The laboratory study showed that gross nitrification decreased with decreasing water potential and was more sensitive to drying than ammonification in the Oa horizon; however, this was not found in the field experiment. The latter might result from the low rates and huge spatial variation, indicating the difference between disturbed samples and intact soil cores. No rewetting pulse of gross ammonification was observed, probably due to its short duration or due to the slow changes of the water potential during the natural rewetting. Although the in situ fluxes of DIN increased at the throughfall exclusion plots after rewetting, the cumulative DIN flux at the throughfall exclusion plots did not significantly exceed that at the control plots. The lowest fluxes of DON and DOC were observed at the throughfall exclusion plots because of the reduction of input with throughfall. In the studies presented here, extended drought periods caused a reduction of gross N turnover in forest soils but gross ammonification continued at considerable rates at low water potential. The hypothesis of increased N turnover and fluxes of DIN, DON and DOC as a consequence of drying/rewetting was not confirmed. In the fen site, undisturbed soil cores were taken and divided to two treatments of water table: permanently flooded and fluctuated. The later was subjected to flooding, drawdown and re-flooding. The permanently flooding enhanced gross ammonification after a lag phase of about 30 days while CO2 emissions were constantly low. The water table drawdown also increased gross ammonification, but again after a lag phase of about 30 days. The first peak of CO2 emissions appeared immediately after water table drawdown, followed by a decrease and a second peak. The ratio of CO2 emission/gross ammonification were close to 2 under anoxic condition which seems to be caused by fast N turnover in the microbial biomass-N pool and low rates of CO2 production. The changes induced by water table drawdown on the N and C turnover were found reversible after re-flooding. Drainage increases SO42- but decrease Fe, DON and DOC concentrations and vice versa when the soils were flooded. Release of DON and DOC was inhibited by increasing SO42- concentrations. Under field conditions, neither drainage nor flooding had an effect on dissolved inorganic N due to the low concentration, indicating the rapid consumption of mineralized N in the field. In the absence of plant uptake and runoff in the laboratory experiment, however, NH4+ increased during the flooding period. Soil desiccation affects the upper soil layers with largest rates of N turnover. While gross N turnover is reduced by soil desiccation, a substantial rate of ammonification was observed even at low water potentials. Nitrification was found more sensitive to desiccation than ammonification which might change the NH4/NO3 ratio of available N under dry conditions. Rewetting of dry soil does not induce a pulse of N turnover and fluxes of DIN, DON and DOC. Overall, an increasing frequency of drying/rewetting cycles seem to have only moderate effect on the N turnover and on N solute fluxes in forest soils. Fluctuations of water table play an important role for the organic matter mineralization, soil solution chemistry and inorganic N availability in minerotrophic fen soils. Acidification by oxidation of S to SO42- can be expected after water table drawdown, causing inhibition of DON and DOC release. The effect of drainage and flooding on gross mineralization and solute concentrations is reversible within a month period. The effect of changing water table regime on N and C turnover in fen soils seems to depend largely on the time scale of the fluctuations. Short term fluctuations at a daily scale will have little effect on N turnover as compared to longer term changes on a monthly scale, while short term changes seem to trigger C losses by CO2.