Soil nitrogen cycle unresponsive to decadal long climate change in a Tasmanian grassland

Increases in atmospheric carbon dioxide (CO2) and global air temperature affect all terrestrial ecosystems and often lead to enhanced ecosystem productivity, which in turn dampens the rise in atmospheric CO2 by removing CO2 from the atmosphere. As most terrestrial ecosystems are limited in their productivity by the availability of nitrogen (N), there is concern about the persistence of this terrestrial carbon sink, as these ecosystems might develop a progressive N limitation (PNL). An increase in the gross soil N turnover may alleviate PNL, as more mineral N is made available for plant uptake. So far, climate change experiments have mainly manipulated one climatic factor only, but there is evidence that single-factor experiments usually overestimate the effects of climate change on terrestrial ecosystems. In this study, we investigated how simultaneous, decadal-long increases in CO2 and temperature affect the soil gross N dynamics in a native Tasmanian grassland under C3 and C4 vegetation. Our laboratory 15N labeling experiment showed that average gross N mineralization ranged from 4.9 to 11.3 µg N g−1 day−1 across the treatment combinations, while gross nitrification was about ten-times lower. Considering all treatment combinations, no significant effect of climatic treatments or vegetation type (C3 versus C4 grasses) on soil N cycling was observed.

Proteolytic Activity in Meadow Soil after the Application of Phytohormones

Phytohormones, similar to soil enzymes, are synthesized and secreted into the soil environment by fungi and microorganisms. Phytohormones are involved in regulating microbial community activity in the rhizosphere. This paper examines how auxins, cytokinins, ethephon and chlorocholine chloride affect the activity of native soil proteases in the organo-mineral horizon of an alpine meadow. In the meadow habitat, native soil proteases were inhibited by auxins whereas the effect of cytokinins on these enzymes was not statistically significant. A similar inhibitory effect on the activity of proteases was shown for ethephon and chlorocholine chloride, both of which also inhibited the activity of native soil proteases in the alpine meadow soil. Overall, the inhibitory effect of phytohormones on the activity of native protease activity may affect plant nutrition by retarding the nitrogen cycle in the soil. This work contributes to our understanding of the influence of substances produced by the rhizosphere that can actively participate in the activity of soil microorganisms and consequently influence the soil nitrogen cycle.

Effect of thinning on the amount of mineral nitrogen

The soil nitrogen cycle and the dynamics of its transformation are closely related to the functioning of the forest ecosystem. This cycle, and the availability of nitrogen as a necessary nutrient in the soil, can be influenced by the process of thinning. The aim of this study is to describe the impact of silvicultural measures on the content of ammonium and nitrate nitrogen in forest soil. Attention is paid to the organic (spruce treatments) and organomineral horizon (beech treatments) in which the transformation of soil nitrogen is most pronounced. Spruce treatments at the Rájec-Němčice area and beech stands at the Březina area, both in the region of Drahanská vrchovina (Czech Republic), were selected for the experiments. Two variants of thinning thinning from below and thinning from above, were performed in the spruce treatments, and thinning from above was performed in the beech treatments. Control variants with no silvicultural measures were defined in both treatments. The amount of ammonium nitrogen in the spruce treatments with thinning from above was in most cases higher than in the other variants. On the contrary, in variant with thinning from below, the ammonium nitrogen content decreased. In terms of the nitrate nitrogen content, the values were generally higher for variants with silvicultural measures than for the control variants. In the beech treatments, the amount of ammonium nitrogen increased and, on the contrary, there was a small decrease in the amount of nitrate nitrogen due to the effect of thinning from above. The differences between thinning from above and the control variants in the beech treatments were less noticeable than in the spruce treatments. Overall, however, it can be said that the nitrogen content available to the vegetation increased. The results of the given experiment provide insight into the trends of nitrogen mineralization intensity in stands in which silvicultural measures are performed.

Effects of highly volatile organochlorine solvents on nitrogen metabolism and microbial counts

The effects of highly volatile organochlorine solvents (1,1,1-trichloroethane, TCET; trichloroethylene, TCE; and tetrachloroethylene, PCE) on soil nitrogen cycle and microbial counts were investigated using volcanic ash soil with different fertilizations. All the solvents significantly inhibited the activity of the cycle under the sealed conditions with 10 to 50 mg/g (dry soil) solvents added. No significant difference between the solvents, and between fertilization plots, was observed. Nitrate ion was not accumulated, and instead, ammonium ion was highly accumulated in the presence of the solvents. Nitrite ion was partially detected, while l-glutaminase activity was inhibited. The growths of ammonification, nitritation, nitratation and denitrification bacteria, and filamentous fungi were significantly inhibited in the presence of 10 mg/g (dry soil) of the solvents.