ABSTRACT The activation energy ( E a ) is the minimum energy necessary for (bio)chemical reactions acting as an energy barrier and defining reaction rates, for example, organic matter transformations in soil. Based on the E a database of (i) oxidative and hydrolytic enzyme activities, (ii) organic matter mineralization and CO 2 production, (iii) heat release during soil incubation, as well as (iv) thermal oxidation of soil organic matter (SOM), we assess the E a of SOM transformation processes. After a short description of the four approaches to assess these E a values—all based on the Arrhenius equation—we present the E a of chemical oxidation (79 kJ mol −1 , based on thermal oxidation), microbial mineralization (67 kJ mol −1 , CO 2 production), microbial decomposition (40 kJ mol −1 , heat release), and enzyme‐catalyzed hydrolysis of polymers and cleavage of mineral ions of nutrients (33 kJ mol −1 , enzyme driven reactions) from SOM. The catalyzing effects of hydrolytic and oxidative enzymes reduce E a of SOM decomposition by more than twice that of its chemical oxidation. The E a of enzymatic cleavage of mineral ions of N, P, and S from their organic compounds is 9 kJ mol −1 lower (corresponding to 40‐fold faster reactions) than the hydrolysis of N‐, P‐, and S‐free organic polymers. In soil, where organic compounds are physically protected and enzymes are partly deactivated, microbial mineralization is ~140‐fold faster compared to its pure chemical oxidation. Because processes with higher E a are more sensitive to temperature increase, global warming will accelerate the decomposition of stable organic compounds and boost the C cycle much stronger than the cycling of nutrients: N, P, and S. Consequently, the stoichiometry of microbially utilized compounds in warmer conditions will shift toward organic pools with higher C/N ratios. This will decouple the cycling of C and nutrients: N, P, and S. Overall, the E a of (bio)chemical transformations of organic matter in soil enables to assess process rates and the inherent stability of SOM pools, as well as their responses to global warming.
Although soil structure is essential for land sustainability, resilience, and fertility, the biological factors of aggregate formation and dynamics, especially the role of mycorrhizal fungi, are still unclear. Soils from a secondary succession, grasslands → shrublands → secondary forests → primary forests in subalpine regions, were sampled to compare the contribution of mycorrhizal fungal communities to the aggregate structure. Four aggregate size classes (>2000 and 250–2000 µm, large and small macroaggregates, respectively; 53–250 µm microaggregates; and < 53 µm silt and clay fraction) were related to microbial community structure, root biomass, and morphology, mycorrhizal infection rate, and external mycorrhizal hyphae. The proportion of large macroaggregates increased by 10 %, while small macroaggregates decreased by 9 % during vegetation succession, increasing the mean weight diameter by 17 %. The Shannon–Wiener and Chaos indexes of arbuscular mycorrhizal fungi (AMF) in grassland soil were three times and in shrubland, they were 24 times higher than in the primary spruce forests. The Simpson index of AMF had a positive linear relationship with soil structure. The mycorrhizal hyphal density was maximal in primary forests and could explain the aggregate size distribution before the arboreous layer developed. In contrast, dissolved organic carbon (DOC) and the diameter of the mycorrhizal hyphal contributed to the distribution of aggregates during later succession stages. Thus, biological aggregation mechanisms in the studied subalpine region followed ecosystem succession, with the mycorrhizal fungal communities being the crucial factor affecting the soil structure mainly during grassland and shrubland succession (AMF stage). In contrast, during the ECM stage, the contribution of DOC to aggregate formation became more critical.
Silicon (Si) is the second‐most abundant element in the earth's crust. In the pedosphere, however, huge spans of Si contents occur mainly caused by Si redistribution in soil profiles and landscapes. Here, we summarize the current knowledge on the different pools and fluxes of Si in soils and terrestrial biogeosystems. Weathering and subsequent release of soluble Si may lead to (1) secondarily bound Si in newly formed Al silicates, (2) amorphous silica precipitation on surfaces of other minerals, (3) plant uptake, formation of phytogenic Si, and subsequent retranslocation to soils, (4) translocation within soil profiles and formation of new horizons, or (5) translocation out of soils (desilication). The research carried out hitherto focused on the participation of Si in weathering processes, especially in clay neoformation, buffering mechanisms for acids in soils or chemical denudation of landscapes. There are, however, only few investigations on the characteristics and controls of the low‐crystalline, almost pure silica compounds formed during pedogenesis. Further, there is strong demand to improve the knowledge of (micro)biological and rhizosphere processes contributing to Si mobilization, plant uptake, and formation of phytogenic Si in plants, and release due to microbial decomposition. The contribution of the biogenic Si sources to Si redistribution within soil profiles and desilication remains unknown concerning the pools, rates, processes, and driving forces. Comprehensive studies considering soil hydrological, chemical, and biological processes as well as their interactions at the scale of pedons and landscapes are necessary to make up and model the Si balance and to couple terrestrial processes with Si cycle of limnic, fluvial, or marine biogeosystems.
No abstract is provided for this article.
Indonesia´s forest is ranked among the Amazonian and the Kongo Basin as the largest tropical rainforest area worldwide. However, the country experiences a severe forest loss since the 1970s. Besides a growing population, the primary pressures are export-orientated timber production and a global commodity demand that lead to a permanent conversion from forest to agricultural areas. The roles of resulting transformation systems of tropical riparian rainforests for ecological functions have yet received little attention in scientific research. Especially C stocks in riparian zones are strongly affected by climate and land use changes that lead to changes in water regime and ground water level drops. We investigated the effects of land transformations in riparian ecosystems of Sumatra, on soil C content, stocks and decomposability. C losses in rubber and oil palm plantations and rainforests were compared and the contribution of soil erosion and organic matter mineralization was estimated. Based on δ Carbon-13 along soil depth, two processes decreasing C stocks were distinguished: erosion and mineralization of soil organic matter (SOM). Depending on the shift of the δ Carbon-13 value of SOC in the topsoil from the linear regression calculated by δ Carbon-13 value with log(SOC) in the topsoil, modification of C turnover rate in the top soil was evaluated. Erosion was estimated by the shift of the δCarbon-13 value of SOC in the subsoil under plantations. The Ah-horizons in non-riparian soils under oil palm and rubber plantations showed with 70% and 62 % a strong reduction in C content and a strong erosion: 35 ±8 cm in oil palm and 33 ±10 cm in rubber plantations. Within the riparian zones an inhomogenous spatial distribution of C content is expected, due to the trend of increasing C stocks from terrestrial through semi –terrestrial to wet conditions. By comparing decreasing δ Carbon-13 values of SOC in the topsoil to those in subsoil, a lower erosion in all transformation systems in riparian zones could be detected.
Improving soil organic carbon (SOC)&#160;and&#160;total nitrogen&#160;(TN)&#160;stocks in croplands is crucial to mitigate climate change and ensuring food security. Soil microbes are important engines driving terrestrial biogeochemical cycles. Their C&#160;use efficiency (CUE) and N&#160;use efficiency (NUE), defined as the proportion of metabolized organic C and N allocated to microbial biomass, is a key regulator controlling the fate of soil C and N. It is assumed that microbial CUE and NUE will increase with higher organic fertilizers application rates, however, any empirical evidence is scare. Microbial necromass is a large and persistent component of SOC and TN especially under croplands. It is still unclear how the simultaneous cycling of C and N in soils would be affected under longer-term organic fertilizer addition.&#160;Here, we studied soil microbial CUE and NUE simultaneously using 18O-H2O tracer approach in a 13-year winter wheat-summer&#160;maize cropping rotation&#160;field trial&#160;&#160;in the North China&#160;Plain.Here organic fertilizers (i.e. straw, manure, compost, biogas residue and biochar) are annually &#160;applied with (optimal N input, Nopt) or without N (zero N input, N0) addition. We found straw, manure, compost, and biogas residue additions increased microbial CUE by 42-80% in N0&#160;and 40-77% in Nopt, and NUE by 25-65% and 33-127% in both N0&#160;and Nopt, respectively. Organic matter addition increased the SOC by 36-150% and 31-137% in both N0&#160;and Nopt, while the TN increased by 21-55% and 21-70% in both N0&#160;and Nopt, respectively. Additionally, organic material additions increased the total microbial necromass C and N by 50-83%, 60-97% and 38-80%, 52-93% under N0&#160;and Nopt, which contributed 14-43% and 33-58% to the SOC and TN, respectively. We concluded&#160;that these C-induced enhancements in microbial growth and CUE or NUE and necromass accumulation were mainly owing to an increased C availability (Easily oxidizable organic C, EOC), and fungal fast-growth strategists.&#160;Composting most effectively facilitated microbial C&#160;and N&#160;cycling, promoting SOC&#160;and TN&#160;accumulation in cropland.
No abstract is provided for this article.
No abstract is provided for this article.
http://dx.doi.org/10.13039/501100014219 National Science Fund for Distinguished Young Scholars
An artificial soil was produced by open air composting of a mixture of sewage sludge, wood shavings, green compost, and a sandy loam soil, and used to investigate the effect of plant growth (wheat) and organic matter application on turnover of organic C und N. A greenhouse experiment was carried out with and without plants (wheat) to test the turnover of N in artificial soil and a mixture of artificial soil with sand in the ratio of 1:1. The results showed that the cultivation of wheat increased the net N mineralization by about 68% to 71% in comparison to treatments without plants. A laboratory experiment was carried out to investigate the effect of incubation of straw or animal manure with artificial soil at 35 °C on C mineralization and net N mineralization. The addition of straw and animal manure caused an increase in mineralization of C. The net N mineralization was higher after addition of animal manure and without substrate application (control) than after addition of straw.
