Search

Many rivers worldwide are regulated, and the altered hydrology can lead to mass development of aquatic plants. Plant invasions are often seen as a nuisance for human activities leading to costly remedial actions with uncertain implications for aquatic biodiversity and ecosystem functioning. Mechanical harvesting is often used to remove aquatic plants and knowledge of plant growth rate could improve management decisions. Here, we used a simple light-temperature theoretical model to make a priori prediction of aquatic plant photosynthesis. These predictions were assessed through an open-channel diel change in O 2 mass balance approach. A Michaelis-Menten type model was fitted to observed gross primary production (GPP) standardised at 10 °C using a temperature dependence from thermodynamic theory of enzyme kinetics. The model explained 87 % of the variability in GPP of a submerged aquatic plant (Juncus bulbosus L.) throughout an annual cycle in the River Otra, Norway. The annual net plant production was about 2.4 (1.0-3.8) times the standing biomass of J. bulbosus. This suggests a high continuous mass loss due to hydraulic stress and natural mechanical breakage of stems, as the biomass of J. bulbosus remained relatively constant throughout the year. J. bulbosus was predicted to be resilient to mechanical harvesting with photosynthetic capacity recovered within two years following 50-85 % plant removal. The predicted recovery was confirmed through a field experiment where 72 % of J. bulbosus biomass was mechanically removed. We emphasise the value of using a theoretical approach, like metabolic theory, over statistical models where a posteriori results are not always easy to interpret. Finally, the ability to predict ecosystem resilience of aquatic photosynthesis in response to varying management scenarios offers a valuable tool for estimating aquatic ecosystem services, such as carbon regulation. This tool can benefit the EU Biodiversity Strategy and UN Sustainable Development Goals., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier B.V. All rights reserved.)

The long-lived greenhouse gas nitrous oxide (N 2 O) and short-lived reactive nitrogen (N r ) gases such as ammonia (NH 3 ), nitrous acid (HONO), and nitrogen oxides (NO x ) are produced and emitted from fertilized soils and play a critical role for climate warming and air quality. However, only few studies have quantified the production and emission potentials for long- and short-lived gaseous nitrogen (N) species simultaneously in agricultural soils. To link the gaseous N species to intermediate N compounds [ammonium (NH 4 + ), hydroxylamine (NH 2 OH), and nitrite (NO 2 - )] and estimate their temperature change potential, ex-situ dry-out experiments were conducted with three Chinese agricultural soils. We found that HONO and NO x (NO + NO 2 ) emissions mainly depend on NO 2 - , while NH 3 and N 2 O emissions are stimulated by NH 4 + and NH 2 OH, respectively. Addition of 3,4-dimethylpyrazole phosphate (DMPP) and acetylene significantly reduced HONO and NO x emissions, while NH 3 emissions were significantly enhanced in an alkaline Fluvo-aquic soil. These results suggested that ammonia-oxidizing bacteria (AOB) and complete ammonia-oxidizing bacteria (comammox Nitrospira) dominate HONO and NO x emissions in the alkaline Fluvo-aquic soil, while ammonia-oxidizing archaea (AOA) are dominant in the acidic Mollisol. DMPP effectively mitigated the warming effect in the Fluvo-aquic soil and the Ultisol. In conclusion, our findings highlight NO 2 - significantly stimulates HONO and NO x emissions from dryland agricultural soils, dominated by nitrification. In addition, subtle differences of soil NH 3 , N 2 O, HONO, and NO x emissions indicated different N turnover processes, and should be considered in biogeochemical and atmospheric chemistry models., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Soil acidification along with base cations loss degrades soil quality and is a major environmental problem, especially in agroecosystems with extensive nitrogen (N) fertilization. So far, the rates of proton (H + ) production and real soil acidification (loss of base cations) remain unclear in subtropical agricultural watersheds. To assess the current status and future risk of soil acidification in subtropical red soil region of China, a two-year monitoring was conducted in a typical agricultural watershed with upland, paddy fields, and orchards where high N fertilizers are applied (320 kg N ha -1 yr -1 ). H + production, neutralization and base cations losses were quantified based on the inputs (rainwater, inflow of water, and fertilizer) and outputs (outflow of water, groundwater drainage, and plant uptake) of major elements (K + , Ca 2+ , Na + , Mg 2+ , Al 3+ , NH 4 + , NO 3 - , SO 4 2- , Cl - , and H + ). The result showed that total H + production in the watershed was 5152 mol c ha -1 yr -1 . N transformation was the most important H + source (68%), followed by excess plant uptake of cations (25%) and H + deposition (7%). Base cations exchange and weathering of minerals (3842 mol c ha -1 yr -1 ) dominated H + neutralization, followed by SO 4 2- adsorption (1081 mol c ha -1 yr -1 ), while H + and Al 3+ leaching amounted to 431 mol c ha -1 yr -1 , only. These results state clearly that despite significant soil acidification, the acidification of surface waters is minor, implying that soils have buffered substantially the net H + addition. As a result of soil buffering, there was abundant loss of base cations, whose rate is significantly higher than the previously reported weathering rate of minerals in red soils (3842 vs 230-1080 mol c ha -1 yr -1 ). This suggests that the pool of exchangeable base cations is being depleted in the watershed, increasing the vulnerability of the watershed, and posing a serious threat to future recovery of soils from acidification., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Nitrogen (N)-induced soil acidification has received much attention worldwide. Nitrification and soil N mineralization are two key N cycle processes that affect soil acidification. However, the seasonal dynamics of soil pH under their combined influence is unclear. We studied the effect of N fertilization on soil pH and N transformations using 15 N tracing in field lysimeters with soils developed from different parent materials (Quaternary red clay, sandstone, and basalt). Maize was planted with 200 kg N ha -1  yr -1 15 N-labeled urea addition. During 7-45 days after fertilization, proton (H + ) production due to nitrification of fertilizer N, nitrate (NO 3 - ) leaching, and plant uptake exceeded H + consumption by base cations mobilization and leaching, resulting in a significant soil pH decline. When nitrification activity decreased (after 45 days), due to exhausted ammonium (NH 4 + ) availability, soil pH rose again. During the fallow period, acid neutralization due to base cation mobilization, and ammonification of soil organic N (SON) offset H + production caused by nitrification of mineralized SON, leading to a sustained rise in soil pH. After the one-year experiment, no significant soil pH decrease was observed in any of the soils. Parent material had little effect on the seasonal dynamics of soil acidification, which appeared to be controlled by fertilization, environmental factors (temperature and moisture), and plant uptake. In subtropical regions, monitoring of soil pH on an annual basis may mask the effect of N fertilization on soil acidification., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Forest soils in the warm-humid tropics significantly contribute to the regional greenhouse gas (GHG) budgets. However, spatial heterogeneity of GHG fluxes is often overlooked. Here, we present a study of N 2 O and CH 4 fluxes over 1.5 years, along a topographic gradient in a rainforest catchment in Xishuangbanna, SW China. From the upper hillslope to the foot of the hillslope, and further to the flat groundwater discharge zone, we observed a decrease of N 2 O emission associated with an increase of soil water-filled-pore-space (WFPS), which we tentatively attribute to more complete denitrification to N 2 at larger WFPS. In the well-drained soils on the hillslope, denitrification at anaerobic microsites or under transient water-saturation was the potential N 2 O source. Negative CH 4 fluxes across the catchment indicated a net soil CH 4 sink. As the oxidation of atmospheric CH 4 is diffusion-limited, soil CH 4 consumption rates were negatively related to WFPS, reflecting the topographic control. Our observations also suggest that during dry seasons N 2 O emission was significantly dampened (<10 μg N 2 O-N m -2  h -1 ) and CH 4 uptake was strongly enhanced (83 μg CH 4 -C m -2  h -1 ) relative to wet seasons (17 μg N 2 O-N m -2  h -1 and 56 μg CH 4 -C m -2  h -1 ). In a post-drought period, several rain episodes induced exceptionally high N 2 O emissions (450 μg N 2 O-N m -2  h -1 ) in the groundwater discharge zone, likely driven by flushing of labile organic carbon accumulated during drought. Considering the global warming potential associated with both GHGs, we found that N 2 O emissions largely offset the C sink contributed by CH 4 uptake in soils (more significant in the groundwater discharge zone). Our study illustrates important topographic controls on N 2 O and CH 4 fluxes in forest soils. With projected climate change in the tropics, weather extremes may interact with these controls in regulating forest GHG fluxes, which should be accounted for in future studies., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Nitrous oxide (N 2 O) emission from winter oilseed rape (WOSR) cultivation may compromise the sustainability of oilseed rape biodiesel. Typically, greenhouse gas budgets of WOSR cultivation assume an N 2 O emission factor (EF) of 1% of the N added in fertilizer and crop residues. Management options to reduce direct soil emissions of N 2 O include the application of biochar, but efficacy and mechanisms of N 2 O suppression are elusive. We measured N 2 O emissions in a WOSR field trial on a sandy loam soil in Denmark over 402 days in 2017-2018, comparing biochar applications from two feedstocks (wheat straw and pig manure fibers), two application rates (1.5 and 15 Mg ha -1 ) and field ageing of up to three years. Further, a controlled incubation experiment was performed to examine the effect of biochar dose and ageing on N 2 O production and consumption by denitrification. Biochar treatments had no significant effects on cumulative N 2 O emissions (1.71-2.78 kg N ha -1  yr -1 ). Likewise, no significant effects were found on crop yield, yield-scaled N 2 O emission, soil mineral N content, gravimetric soil moisture or pH. The fertilizer induced EF was 0.51% which is well below the IPCC Tier 1 EF of 1%. High doses of fresh, but not field-aged biochar suppressed N 2 O production under anoxic conditions ex situ, suggesting that biochar with sufficient liming capacity could mitigate N 2 O emissions from denitrification also under field conditions. Yet, rates of up to 15 Mg ha -1 flash pyrolysis biochar in the current in situ study, which comprised a pronounced summer drought, showed no significant N 2 O mitigation. This highlights the need for selecting dedicated biochars and doses and test them in multi-year studies to conclude on their N 2 O mitigating effect. Yet, in relation to sustainability of WOSR cultivation for biodiesel, the current study suggests that C sequestration by biochar is not compromised by increased N 2 O emissions., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)