Climate change reduces the net sink of CH\u3csub\u3e4\u3c/sub\u3e and N\u3csub\u3e2\u3c/sub\u3eO in a semiarid grassland

Atmospheric concentrations of methane (CH4) and nitrous oxide (N2O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO2 and temperature affected CH4 and N2O fluxes in a well-drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH4 and N2O fluxes to elevated CO2 and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO2 increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO2 and warming would have opposing effects on CH4 and N2O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell-shaped relationship was observed with soil moisture with highest CH4 uptake at intermediate soil moisture. Both N2O emission and uptake occurred at our site with some years showing cumulative N2O emission and other years showing cumulative N2O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO2 and warming reduced the sink of CH4 and N2O expressed in CO2 equivalents (across 5 years by 7% and 11% for elevated CO2 and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH4 and N2O expressed in CO2-equivalents

Ameliorating soil acidity with calcium carbonate and calcium hydroxide: effects on carbon, nitrogen, and phosphorus dynamics

Calcium carbonate (CaCO3) is frequently used to ameliorate soil acidity in agricultural soils. An alternative is calcium hydroxide (Ca(OH)2), which is more soluble than CaCO3. However, the associated change in soil parameters remains unclear. We aimed to evaluate the different responses of available nitrogen (N) and phosphorus (P), microbial biomass carbon (C), N, and P (MBC, MBN, and MBP), and dissolved organic C and N (DOC and DON) to CaCO3 and Ca(OH)2. We amended an acidic soil (pH of 5.6) with CaCO3 or Ca(OH)2 at 5 different loadings of hydroxide (OH−, 0.025, 0.05, 0.1, 0.25, and 0.75 mmol g−1) in a 30-day incubation experiment. Both Ca(OH)2 and CaCO3 rapidly increased soil pH, but soil pH increased to 10 with the highest loading rate of Ca(OH)2, while soil pH levelled off at 7.5 with an OH− loading of 0.25 mmol g−1 as CaCO3. Higher ammonium and lower nitrate concentration with high OH− loading of Ca(OH)2 (>0.25 mmol g−1) suggests that nitrification was constrained under basic soil conditions, while adding CaCO3 showed the opposite results. Ca(OH)2 addition increased DOC and DON, along with MBC and MBN, suggesting that desorption of organic matter stimulated microbial growth. High OH− loading with CaCO3 reduced available P due to high P-fixation more than Ca(OH)2. Our study shows that while both Ca(OH)2 and CaCO3 can be used to ameliorate soil acidity, Ca(OH)2 may be able to reduce nitrification and thus N loss at high OH− loading and enhance available P more than CaCO3

Disentangling root responses to climate change in a semiarid grassland

Future ecosystem properties of grasslands will be driven largely by belowground biomass responses to climate change, which are challenging to understand due to experimental and technical constraints. We used a multi-faceted approach to explore single and combined impacts of elevated CO2 and warming on root carbon (C) and nitrogen (N) dynamics in a temperate, semiarid, native grassland at the Prairie Heating and CO2 Enrichment experiment. To investigate the indirect, moisture mediated effects of elevated CO2, we included an irrigation treatment. We assessed root standing mass, morphology, residence time and seasonal appearance/disappearance of community-aggregated roots, as well as mass and N losses during decomposition of two dominant grass species (a C3 and a C4). In contrast to what is common in mesic grasslands, greater root standing mass under elevated CO2 resulted from increased production, unmatched by disappearance. Elevated CO2 plus warming produced roots that were longer, thinner and had greater surface area, which, together with greater standing biomass, could potentially alter root function and dynamics. Decomposition increased under environmental conditions generated by elevated CO2, but not those generated by warming, likely due to soil desiccation with warming. Elevated CO2, particularly under warming, slowed N release from C4—but not C3—roots, and consequently could indirectly affect N availability through treatment effects on species composition. Elevated CO2 and warming effects on root morphology and decomposition could offset increased C inputs from greater root biomass, thereby limiting future net C accrual in this semiarid grassland

Carbon efficiency for nutrient acquisition (CENA) by plants: role of nutrient availability and microbial symbionts

Background In a recent framework, Raven et al. (2018) considered carbon cost of acquiring phosphorus by mycorrhizal and non-mycorrhizal plants. Scope We broaden their conceptual framework by incorporating belowground carbon allocation for both nitrogen and phosphorus acquisition in conditions of nutrient co-limitation and shifts in nutrient limitation, symbiotic associations with nitrogen-fixing bacteria, and nutrient mining via rhizosphere priming. We introduce a new parameter: carbon efficiency for nutrient acquisition (CENA) defined as the amount of nutrient acquisition per unit carbon allocated belowground. Results We explain how CENA increases with increased nutrient availability, and how it reaches a plateau when the increased availability of one limiting nutrient leads to the emergence of limitation by another nutrient. We describe how the relationship between CENA and mycorrhizal plants may be less steep compared to non-mycorrhizal plants so that CENA may be higher for mycorrhizal plants when nutrient availability is low and vice versa. In contrast, the CENA of nitrogen-fixing plants would be independent of soil nitrogen availability as long as biological nitrogen fixation meets plant nitrogen demand, but it would increase with increased soil phosphorus availability. The CENA would be more affected by soil nitrogen and phosphorus locked in organic matter or insoluble forms if plants perform nutrient mining strategies, but would be more sensitive to soil nitrogen and phosphorus availabilities if plants rely on nutrient scavenging strategies. Conclusions The updated conceptual frameworks would provide better understanding of how plants optimize belowground carbon allocation for nutrient acquisition that is affected by perturbations in nutrient availability

Disentangling root responses to climate change in a semiarid grassland

Future ecosystem properties of grasslands will be driven largely by belowground biomass responses to climate change, which are challenging to understand due to experimental and technical constraints. We used a multi-faceted approach to explore single and combined impacts of elevated CO2 and warming on root carbon (C) and nitrogen (N) dynamics in a temperate, semiarid, native grassland at the Prairie Heating and CO2 Enrichment experiment. To investigate the indirect, moisture mediated effects of elevated CO2, we included an irrigation treatment. We assessed root standing mass, morphology, residence time and seasonal appearance/disappearance of community-aggregated roots, as well as mass and N losses during decomposition of two dominant grass species (a C3 and a C4). In contrast to what is common in mesic grasslands, greater root standing mass under elevated CO2 resulted from increased production, unmatched by disappearance. Elevated CO2 plus warming produced roots that were longer, thinner and had greater surface area, which, together with greater standing biomass, could potentially alter root function and dynamics. Decomposition increased under environmental conditions generated by elevated CO2, but not those generated by warming, likely due to soil desiccation with warming. Elevated CO2, particularly under warming, slowed N release from C4—but not C3—roots, and consequently could indirectly affect N availability through treatment effects on species composition. Elevated CO2 and warming effects on root morphology and decomposition could offset increased C inputs from greater root biomass, thereby limiting future net C accrual in this semiarid grassland

Drought impacts on tree root traits are linked to their decomposability and net carbon release

Root trait plasticity can facilitate plant adjustment to water shortages, but the impact of altered traits on belowground carbon (C) cycling is mostly unknown. While drought and nutrient availability can alter root morphological and chemical traits that may affect root decomposition, direct assessments of drought mediated changes on decomposability are not available. We exposed four tree species contrasting in drought stress tolerance and root traits to three dry-down and recovery periods (over 5 months after 11 months of growth in well-watered conditions) under high and low nutrient conditions. We then assessed early stage root decomposability in relation to their morphology and chemistry as well as implications for CO2 release when accounting for effects on root biomass. While each species showed a unique set of responses, drought generally reduced root diameter and increased nitrogen concentration. We found limited evidence that morphological responses to drought were counteracted by high nutrient supply. Results indicated that the degree of association between morphological and nutrient root trait responses to drought and decomposability varied with different species. However, across these contrasting woody species, drought-induced increases in nitrogen and phosphorus concentrations were associated with drought-induced increases in early stage root decomposability. When accounting for changes in root biomass, estimated overall C loss through root decomposition increased with drought stress. Our experimental results demonstrate that changes in tree root traits with drought can enhance C loss via root decomposition, and with other factors being equal, drought may potentially contribute to a positive feedback to climate change. Our findings contribute empirical evidence to help disentangle the multiple factors involved in root contribution to C balances at the ecosystem level

Nitrogen and phosphorus availability have stronger effects on gross and net nitrogen mineralisation than wheat rhizodeposition

Soil nitrogen (N) availability is determined by microbial gross N mineralisation (GNM) and immobilisation, where net N mineralisation (NNM) represents their balance. Plants provide a substantial amount of their photosynthesized C belowground into the soil as rhizodeposition, which can stimulate microbial activity affecting GNM and NNM, but this activity also depends on soil N and phosphorus (P) availability. We examined how N (25 and 100 kg N ha 1 or 44 and 177 mg N pot 1) and P (10 and 40 kg P ha 1, or 18 and 71 mg P pot 1) fertilisation affected microbial N mineralisation in soil planted with two wheat genotypes (Suntop and 249) varying in root biomass and rhizodeposition. We used a continuous 13CO2 labelling method to track plant C rhizodeposition and a 15N pool dilution technique to investigate GNM. We further assessed NNM by comparing N pools in plant and soil at the start and end of the experiment. We observed increased GNM with increased P fertilisation, likely because of P-induced N limitation stimulating microbial mining for N, particularly at the low level of N fertilisation. N fertilisation did not affect GNM but the higher level of N fertilisation reduced NNM, likely because of increased microbial immobilisation of fertiliser N. Our results suggest that GNM was more sensitive to soil N and P availability than to rhizodeposition between wheat genotypes, although at high N fertilisation, rhizodeposition contributed to reduced NNM, likely because rhizodeposition enhanced microbial N immobilisation. We conclude that the relative availability of N and P in soil should be considered for managing GNM and NNM in soil

Seasonal Biotic Processes Vary the Carbon Turnover by Up To One Order of Magnitude in Wetlands

Soil Organic Carbon (SOC) turnover t in wetlands and the corresponding governing processes are still poorly represented in numerical models. t is a proxy to the carbon storage potential in each SOC pool and C fluxes within the whole ecosystem; however, it has not been comprehensively quantified in wetlands globally. Here, we quantify the turnover time t of various SOC pools and the governing biotic and abiotic processes in global wetlands using a comprehensively tested process-based biogeochemical model. Globally, we found that t ranges between 1 and 1,000 years and is controlled by anaerobic (in 78% of global wetlands area) and aerobic (15%) respiration, and by abiotic destabilization from soil minerals (5%). t in the remaining 2% of wetlands is controlled by denitrification, sulfur reduction, and leaching below the subsoil. t can vary by up to one order of magnitude in temperate, continental, and polar regions due to seasonal temperature and can shift from being aerobically controlled to anaerobically controlled. Our findings of seasonal variability in SOC turnover suggest that wetlands are susceptible to climate-induced shifts in seasonality, thus requiring better accounting of seasonal fluctuations at geographic scales to estimate C exchanges between land and atmosphere

Soil Microbes Compete Strongly with Plants for Soil Inorganic and Amino Acid Nitrogen in a Semiarid Grassland Exposed to Elevated CO\u3ci\u3e2\u3c/i\u3e and Warming

Free amino acids (FAAs) in soil are an important N source for plants, and abundances are predicted to shift under altered atmospheric conditions such as elevated CO2. Composition, plant uptake capacity, and plant and microbial use of FAAs relative to inorganic N forms were investigated in a temperate semiarid grassland exposed to experimental warming and free-air CO2 enrichment. FAA uptake by two dominant grassland plants, Bouteloua gracilis and Artemesia frigida, was determined in hydroponic culture. B. gracilis and microbial N preferences were then investigated in experimental field plots using isotopically labeled FAA and inorganic N sources. Alanine and phenylalanine concentrations were the highest in the field, and B. gracilis and A. frigida rapidly consumed these FAAs in hydroponic experiments. However, B. gracilis assimilated little isotopically labeled alanine, ammonium and nitrate in the field. Rather, soil microbes immobilized the majority of all three N forms. Elevated CO2 and warming did not affect plant or microbial uptake. FAAs are not direct sources of N for B. gracilis, and soil microbes outcompete this grass for organic and inorganic N when N is at peak demand within temperate semiarid grasslands