Riitta Hyvönen

Effects of acidification and liming on carbon and nitrogen mineralization and soil organisms in mor humus

The aim was to determine if changes in C and N mineralization after acidification and liming could be explained by changes in the soil organism biomass. Intact soil cores from F/H layers in a Norway spruce (C:N=31) and a Scots pine (C:N=44) stand in central Sweden were treated in the laboratory for 55 days with deionized water (control), weak H2SO4 (successively applied as 72 mm of acid rain of pH 3.1), strong H2SO4 (applied as a single high dose of pH 1), and lime CaCO3. Strong acidification reduced C mineralization and increased net N mineralization in both soils. Weak acidification resulted in similar but less pronounced effects. Liming initially stimulated C mineralization rate, but the rates declined, indicating that an easily available C source was successively used up by the microorganisms. Liming also increased net N mineralization in the C:N=31 humus, but not significantly in the C:N--44 humus. Strong acidification generally affected the amounts of FDA-active fungal hyphae, nematodes and enchytraeids more than the other treatments did. The increases in net N mineralization after acidification and liming could only partly be explained by the decreases in biomass N in soil organisms. Mineralization of biomass N from killed soil organisms could at the most explain up to about 30% of the increase in net N mineralization after strong acidification. Most of the effects on N mineralization seemed to depend on the fact that acidification reduced and liming increased the availability of C and N to the microorganisms. Furthermore, acidification seemed to reduce the incorporation of N from dead organisms into the soil organic matter and, thereby, make the N compounds more readily available to microbial decomposition and mineralization.

Decomposition and nutrient release from Picea abies (L.) Karst. and Pinus sylvestris L. logging residues.

Abstract We analyzed the long-term dynamics of the decomposition of different fractions of forest litters by using models derived from a theory on decomposition and element cycling in organic matter. The analysis of decomposition was done (i) by measuring decomposition rates of and nutrient changes in needles, twigs, and branches in field experiments, and (ii) by estimating parameters used in the models with information derived from these experiments. The analysis showed that variability in decomposition rate decreases with increasing substrate diameter. We also used the models to predict the long-term dynamics of carbon, nitrogen, and phosphorus in logging residues. Our predictions suggest that from a short-term perspective the nutrient-rich needles and twigs are a more important nutrient source for the subsequent forest generation compared with branches. However, in the long run the nutrient concentration of the coarse litter fractions will also be important. The predicted amounts of carbon and nitrogen in logging residues were compared with measured amounts in humus layer. On a productive Norway spruce site remaining logging residues were, 16 years after clear-felling, predicted to increase carbon amounts in the forest floor by 50% and on a low productive Scots pine site by 100%. The corresponding nitrogen amounts in the forest floor should have been 30% higher at the spruce site and 70–80% higher at the pine site.

Effects of lumbricids and enchytraeids on nematodes in limed and unlimed coniferous mor humus

In a factorial laboratory experiment, specimens of Dendrobaena octaedra (Lumbricidae) and Cognettia sphagnetorum (Enchytraeidae) were added to microcosms with unlimed (pH 4.5) and limed (pH 5.5) coniferous mor humus containing bacteria, fungi, protozoans, and nematodes. Effects on the nematodes were assessed after an incubation period of 207 days at 15°C and a soil moisture content of 60% water-holding capacity. When D. octaedra was absent, nematodes were significantly more abundant in the limed humus than in the unlimed humus. The presence of D. octaedra markedly reduced the number of nematodes in the limed humus but not in the unlimed one, where D. octaedra lost weight and probably did not feed. Most nematodes (92–97%) were bacterial-feeders. The presence of D. octaedra did not decrease the number or biomass of bacteria, indicating that the reduction in nematode numbers was not the result of competition for bacteria between D. octaedra and the nematodes. The presence of C. sphagnetorum had no effect on the nematodes in either of the treatments. We suggest that the reason why D. octaedra, but not C. sphagnetorum, reduced nematode numbers is that the former was more likely to inadvertently ingest the nematodes because of its much greater size. The results provide a possible explanation for the observation that liming sometimes enhances nematode populations, when lumbricids do not respond to the treatment, and sometimes causes decreases, when lumbricids increase in number.

Effects of fungivorous and predatory arthropods on nematodes and tardigrades in microcosms with coniferous forest soil

Effects of fungivorous and predatory soil arthropods on free-living nematodes and tardigrades were studied in a factorial microcosm experiment. A stepwise increase in faunal complexity was obtained by adding soil arthropods to defaunated humus samples from an irrigated+fertilized and an untreated stand of Scots pine. The effects were assessed after 103 and 201 days at 15°C and a soil moisture content of 50% water-holding capacity. The study showed that a diverse community of “fungivorous” arthropods (collembola and oribatid mites), present in numbers similar to those in the field, reduced the abundance of nematodes. A complete community of fungivorous and predatory arthropods (e.g., gamasides, spiders, and cantharid larvae) further strengthened this repressive effect. Certain nematode genera were more affected than others. Tardigrades seemed to be efficient predators on nematodes, but their numbers were, in turn, strongly reduced by predatory arthropods. Because predatory arthropods fed on both nematodes and their tardigrade predators, the impact of arthropod predators on nematode regulation was greater than it appeared to be on the basis of nematode numbers. Humus type also interacted with the other factors. Nematode numbers were initially higher in the untreated humus than in the irrigated+fertilized humus. However, because tardigrade populations increased only in the untreated humus, nematode numbers decreased more in this humus than in the irrigated+fertilized humus. The study demonstrates that nematode abundance can be regulated by a number of types of interacting predators.

Modelling Long‐Term Carbon and Nitrogen Dynamics in an Arable Soil Receiving Organic Matter

Predictions of a model based on a general theory of decomposition of organic matter were compared with measured changes in total soil C and N pools in a 35-yr field experiment on clay loam in central Sweden with biannual additions of straw, peat, sawdust, farmyard manure, green manure, and sewage sludge in order to study which litter char- acteristics determine decomposition and accumulation rate of soil organic matter. The central concepts of the theory are a continuously changing substrate quality, a constant decomposer efficiency, and a climatically controlled decomposer growth rate. The model requires seven parameters to describe C dynamics and two more to describe N dynamics. Substrate qualities were estimated from measured decomposition rates, but no single characteristic of the examined substrates could be considered as a general key indicator on decomposition rate. Predicted values were within a few percent of the measured values for C and within 13% of the measured values for N, although the residual fractions of added C ranged from 14 to 69%, and the residual fractions of N varied from 24 to 400% (net immobilization). In agreement with the field observations, the model predicted that the accumulation of soil C would be highest in the plots receiving peat and lowest in the plots receiving straw and green manure. Intermediate accumulations- were predicted in plots receiving sewage sludge, sawdust, and farmyard manure. The model also predicted that the net contribution of N to the crops from straw, peat, and sawdust would be small or negligible. By contrast, con- tributions of N from sewage sludge and from green and farmyard manure were predicted to be considerable.

Carbon balance of a forest ecosystem after stump harvest

Abstract Stump harvest in forests can cause both reductions of CO2 emissions through a decrease of decomposable substrate (direct effect) and emission increases as a consequence of deep and extensive soil disturbance (indirect effect). Here, the effects of stump harvest on net ecosystem CO2 exchange (NEE) in a former Norway spruce stand in mid Sweden are presented. CO2 exchange was continuously followed by eddy-covariance measurements during the first years after harvest. Differences in NEE from stump harvested and mounded (reference) plots were determined by soil-surface respiration measurements. Respiration from decaying stumps was estimated by a decomposition model. Fluxes indicated a direct effect (decreased efflux) during the first year after harvest that corresponded to the absence of decomposing stumps. During the following years, this emission reduction was increasingly counteracted by an indirect effect (increased efflux) of similar magnitude. This means that the expected emissions caused by extra soil disturbance occur with a certain delay and seem to increase with time. By these emissions, the substitution efficiency of stumps as bioenergy resource is reduced. Furthermore, at a time scale of centuries, instant combustion of stumps leads to a larger contribution to global warming than slow decomposition, because the stump carbon is available earlier in form of greenhouse gas. This is estimated by the time integral of emissions. Thus, despite the surprisingly low initial emissions, the overall substitution efficiency and climate benefits of stump harvest are likely to be small. The long-term consequences of stump harvest for the carbon budget are, however, still uncertain.

Dynamics of soil C, N and Ca in four Swedish forests after removal of tops, branches and stumps as predicted by the Q model

Abstract We used the Q model to examine the dynamics of carbon (C), nitrogen (N) and calcium (Ca) in the litter/soil system in different scenarios of harvesting intensities, S (stems only), SSl (stems and slash, i.e. tops, and branches including needles) and SSlSt (stems, slash and stumps including coarse roots). Empirical data from long-term field experiments in Sweden, two sites with Norway spruce and two with Scots pine with different levels of productivity, were used to calibrate the model against the stem-only treatment. The highest initial reduction in soil C, N and Ca stores was predicted for SSlSt, and the reduction was more pronounced at low productive sites than at the high productive ones. Most of the decline in soil C and Ca stocks was offset by the litter production in the following forest stand. N showed an initial phase of immobilisation in stumps and coarse roots, while N was immediately released from tops and branches, which contained N-rich needles. Removal of stumps and coarse roots in combination with slash resulted in a similar load of inorganic soil N as for the S treatment, whereas the SSl treatment with stumps left in the soil initially reduced the inorganic soil N pool.

Modelling carbon dynamics in coniferous forest soils in a temperature gradient

Climate change and changes in land use will alter the stores of carbon and turnover of soil organic matter. We have used a theory for carbon cycles in terrestrial ecosystems to analyse changes in soil organic matter turnover in coniferous forests. The central concepts of the theory are a continuously changing substrate quality, a constant decomposer efficiency and a climatically controlled decomposer growth rate. Measurements on litter production and soil carbon stores from field experiments have been used to successfully validate the model predictions. Measured litter production increased with increasing temperature but the response was not identical for forests of different vegetation types which reflect variations in productivity. The temperature response of needle-litter production and decomposition rate were strongest in the most productive forests and weakest for the low productive forests. Initial decay rates of soil C store from steady state showed the same trend in temperature response as decay of a single litter cohort did, but the absolute values are 16% of the decay rates of a single litter cohort. Predicted soil C ranged from 5 to 9 kg C m−2. There exists a remarkable variation in forest soil C store response to temperature; the magnitude and even the sign depends on productivity as defined by vegetation type. The assumption that, in general, decomposition rates increase more than NPP with temperature, and consequently, soil C stores should decrease in response to a climate warming, seems therefore too simplistic.

Projecting soil fauna influence on long-term soil carbon balances from faunal exclusion experiments

Abstract In soil ecology, many investigations of faunal influence on, e.g. soil carbon flows have been performed. However, analysis of long-term effects of faunal activity on, e.g. long-term soil carbon pool changes are uncommon. We analyse possible effects on long-term soil carbon balances of soil fauna activity on humus and litter decomposition rates as well as litter humification ratio (the fraction of litter that eventually becomes humus). Results from published soil fauna experiments (measurements made in presence versus absence of organisms) are re-interpreted as parameter changes for a soil carbon model (ICBM, see http://www.mv.slu.se/vaxtnaring/olle/ICBM.html ), which is used for projections of soil C pools and fluxes during a 30-year period. Model outputs indicate that changes in humification ratio and old material (“humus”) decomposition rates have much greater influence than changes in young material (“litter”) decomposition rate on total soil carbon dynamics. We point out the risk of putting too much faith in model projections, and underline the need for long-term research data as a base for long-term model projections.