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Limited nutrient availability is one of the major challenges in organic farming. Little is known about nutrient budgets of organic farms, the underlying factors or effects on soil fertility. We therefore assessed farm gate nutrient budgets for nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg) and sulfur (S) of 20 organic farms in Germany and analyzed their soil nutri-ent status. In average, the budgets showed a surplus of N (19 kg ha−1), K (5 kg ha−1), S (12 kg ha−1), and Mg (7 kg ha−1), and a deficit of P (−3 kg ha−1). There was, however, high variability between farms (e.g. standard deviation up to ± 36 kg N ha−1), which was mainly explained by different degrees of reliance on biological N fixation (BNF) as N source. When farms obtained more than 60% of their N input through BNF, they had deficits of P (mean −8 kg P ha−1) and K (mean −18 kg K ha−1). Nutrient status of most soils was within the ad-vised corridor, but for P, K and Mg, 10–15% of fields were lower and 45–63% were higher than advised. Extractable soil nutrient contents did not correlate with the nutrient budgets, inputs or outputs. Only extractable soil P increased with increasing P inputs and outputs. Fur-thermore, a decrease in extractable soil P was detected with a prolonged history of organic farming, indicating a risk of soil P mining in organic farming systems. In conclusion, the study revealed nutrient imbalances in organic farming and pointed to P and K scarcity as a major challenge for organic farms with high reliance on BNF in the long term.

In order to assess the progress toward eutrophication management goals, it is important to understand trends in land-based nutrient use. Here we present net anthropogenic nitrogen and phosphorus inputs (NANI and NAPI, respectively) for 2000 and 2010 for the Baltic Sea watershed. Overall, across the entire Baltic, between the 5-year periods centered on 2000 and 2010, NANI and NAPI decreased modestly by –6 and –4%, respectively, but with substantial regional variation, including major increases in the Gulf of Riga drainage basin (+19 and +58%, respectively) and decreases in the Danish Straits drainage basin (–25 and –40% respectively). The changes were due primarily to changes in mineral fertilizer use. Mineral fertilizers dominated inputs, at 57% of both NANI and NAPI in 2000, increasing to 68 and 70%, respectively, by 2010. Net food and feed imports declined over that period, corresponding to increased crop production; either fewer imports of food and feedstocks were required to feed humans and livestock, or more of these commodities were exported. A strong linear relationship exists between regional net nutrient inputs and riverine nutrient fluxes for both periods. About 17% of NANI and 4.7% of NAPI were exported to the sea in 2000; these relationships did not significantly differ from those for 2010. Changes in NANI from 2000 to 2010 across basins were directly proportional rather than linearly related to changes in total N (TN) fluxes to the sea (i.e., no change in NANI suggests no change in TN flux). Similarly, for all basins except those draining to the Baltic Proper, changes in NAPI were proportional to changes in total P (TP) fluxes. The Danish Straits decreased most between 2000 and 2010, where NANI and NAPI declined by 25 and 40%, respectively, and corresponding fluxes of TN and TP declined 31 and 18%, respectively. For the Baltic Proper, NAPI was relatively unchanged between 2000 and 2010, while riverine TP fluxes decreased 25%, due possibly to lagged effects of fertilizer reduction resulting from socio-political changes in the early 1990s or improvements in sewage treatment capabilities. For most regions, further reductions in NANI and NAPI could be achieved by more efficient production and greater substitution of manure for imported mineral fertilizers.

Increasing nutrient inputs driven by global environmental pressures may lead to changes in prokaryotic plankton biomass and community composition in coastal environments. Nutrient addition experiments (inorganic, organic, and mixed) were performed at the continental shelf outside the Ría de Vigo, in spring, summer and autumn 2014, and the results contrasted with those from similar previous experiments carried out inside the ría in 2013. The mixed nutrient additions caused the greatest changes in community composition, mostly consisting of blooms of Vibrionales. Inorganic nutrients yielded increased proportions of Oceanospirillales and Alteromonadales. Euclidean distances among samples were used to estimate compositional resistance to disturbances derived from nutrient additions. Changes in prokaryotic biomass were used as an indicator of biomass resistance. Generally, prokaryotic communities were more resistant to inorganic than organic nutrient additions. Communities sampled inside the ría, presumably exposed to greater perturbations, showed milder variability in the non-amended control than those from outside the ría. By contrast, shelf communities, with higher ambient organic matter concentrations, were more resistant to organic and mixed nutrient additions than those from the ría. Our data suggest that the perturbation history is related to the resistance of microbial communities to changes in nutrient inputs into the coastal ocean.

Chemical fertilizer has contributed significantly in increasing food grain production in India. However, there are emerging concerns of environmental pollution at local scale, climate change at global scale, and sustainability of chemical fertilizer-dependent agriculture. Budgeting of nutrient is a valuable tool in assessing the nutrient use efficiency, nutrient mining, and environmental pollution. We constructed a field level top-down nutrient budget for food grain production in India since the onset of the Green Revolution in the country, i.e., 1970 to 2018, using equation-based empirical methods. Total nutrient input to Indian agriculture was 666.4 million tons (Mt) of N, 189.1 Mt of P, and 244.8 Mt of K during 1970–2018. Chemical fertilizer contributed 68.1% of N, 91.3% of P, and 28.8% of K towards the inputs. Nutrient budget for the last 48 years showed that there was positive balance of N (12.2 Mt), accumulation of P (11.7 Mt) but negative balance for K (157.9 Mt). Further, with the business-as-usual scenario, there would be positive balance of 276.2 Mt N, accumulation of 20.9 Mt P, and negative balance of 202 Mt K from Indian agriculture soils by 2050. The nutrient budget provides valuable information on the present status and balance of nutrient use and the trends with time, which will be helpful for reorienting the fertilizer use policies for sustainable agriculture. Graphical abstract

Food sources and food web structure in seagrass meadows are important determinants of ecosystem functions and services. However, there is little information on the effect of eutrophication on food source contributions and food web structure in seagrass meadows. Here we used stable isotopes of carbon and nitrogen (δ13C and δ15N) to investigate how do different levels of nutrient enrichment affect the diets of consumers and food web structure within tropical seagrass meadows. We found that the diet contributions of macroalgae (mean 24% ± 12% for fish, 21% ± 5% for invertebrates), particulate organic matter (mean 19% ± 12% for fish, 18% ± 8% for invertebrates), and sediment organic matter (mean 24% ± 13% for fish, 21% ± 8% for invertebrates) to fish and invertebrates were all higher in seagrass meadows with higher nutrient concentrations, while seagrass and epiphytes contributed more to consumers in seagrass meadows with lower nutrient concentrations. Meanwhile, higher nutrient concentrations decreased the trophic position (mean 2.6 ± 0.5 in high‐nutrient level, 3.4 ± 0.6 in low‐nutrients level) of consumers and food chain length (2.5 in high‐nutrient level, 2.9 in low‐nutrients level). Higher nutrient concentrations reduced the contribution of seagrass carbon to consumers through the grazing food chain, but enhanced the flow of macroalgal carbon to consumers through the grazing food chain. Overall, eutrophication modified the food web structure of seagrass meadows. We recommend that measures be taken to decrease nutrient input into seagrass ecosystems to maintain its important functions and services.

Background Nutrient cycle in tropical forests is mainly driven by litter fall amounts and by litter decay due to litter physical and chemical properties. Apart from differences in site conditions, different successional stages of tropical rainforests may influence nutrient inputs to the soil via differences in litter fall and decay. Methods We studied leaf litter fall, decomposition and related nutrient input to the soil in two secondary tropical rainforest types differing in land-use intensity and history in the lowlands of Sumatra (Indonesia). Results Mean annual litter fall was by 29.5% higher in the old than in the young secondary forests. In contrast, annual litter decomposition differed only marginally (54.9% vs. 52.1%) in young vs. old secondary forests. Litter chemical composition (C, N, P, K contents) did not differ between the forest types. Litter decomposition was also not different between the forest types (4.6% vs. 4.3% loss of initial weight per month). Consequently, annual nutrient input to the soil was primarily affected by differences in leaf litter production, thus, old secondary forests exceeded that of young secondary forests by 30% and 24% for C and N, but was not different regaring P and K among the two forest types. Conclusions The results indicate that litter nutrient inputs to the soil in the two types of secondary tropical lowland forests are most strongly influenced by aboveground litter production rather than litter chemical quality or litter decay. We conclude that the restoration status of secondary tropical lowland forests is crucial for the nutrient status of these forest ecosystems.

Sandy beaches, which represent the most common type of land–sea interface, harbor distinctive biotic communities and regulate the flow of energy between marine and terrestrial ecosystems. Accumulations of sea wrack on sandy beaches are of crucial importance for recycling beach nutrients and for regulating trophic connectivity and coastal functioning. We investigated the role of beaches as biogeochemical hotspots by examining the metabolic activity in accumulations of different species of wrack on two exposed beaches affected by different levels of human pressure. Experimental wrack patches provided large amounts of different sedimentary nutrients over time due to remineralization of the algae. Unsurprisingly, the variation in the nutrients present in the beach sediments was related to the species of wrack considered. Macroalgal wrack was metabolically very active and supported high respiration rates represented by intense CO₂ fluxes. Importantly, we demonstrated that the wrack metabolic rate differed significantly depending on the algal species considered. Different macrofauna and bacterial assemblages were identified in the different wrack patches and on the different beaches. We suggest that human activities such as beach grooming can modify the wrack-associated communities, thus contributing to the variability in the biogeochemical processes and metabolic rates. Significant changes in the type and amount of wrack deposited on beaches can change fundamental processes related to the marine-terrestrial transfer of nutrients and energy and to the marine-atmospheric transfer of CO₂ emissions, with ecological consequences for nearshore environments.

Land-use change from forest to cocoa agroforestry and other tree-based farming systems alters the structure of forest stands and influences the magnitude of canopy water fluxes and subsequent bio-element inputs to the forest floor. The partitioning of incident rainfall (IR) into throughfall (TF), stemflow (SF) and canopy interception loss (ILC) and their associated nutrient element concentrations and fluxes was examined along a replicated chrono-sequence: forest, 3, 15 and 30-year-old smallholder shaded-cocoa systems in Ashanti Region, Ghana. Mean annual precipitation during the 2-year observational period (2007 and 2008) was 1376.2 ± 93.8 mm. TF contributed between 76.5–90.4%, and SF between 1.4–1.7% of the annual IR to the forest floor. There were significant differences in IR, TF and SF chemistry. While TF and SF were enriched in phosphorus (1.33–5.67-fold), potassium (1.1–5.69 fold), calcium (1.35–2.65 fold) and magnesium (1.4–2.68 fold) relative to IR, total N (NH4++NO3−) declined (0.5–0.91) of IR values in TF and SF in forest and shaded cocoa systems. Incident rainfall was significantly more acidic than TF and SF in both forest and shaded-cocoa systems. Mean annual total N, P, K, Ca and Mg inputs to the forest floor through IR were 5.7, 0.14, 13.6, 9.43 and 5.6 kg ha−1year−1 respectively. Though an important source of available nutrients for plant growth, incident rainfall provides only a small percentage of the annual nutrient requirements. With declining soil fertility and pervasive low cocoa yields, possible effects of the reported nutrient fluxes on nutrient budgets in cocoa systems merit further investigation. Against the background of increased TF and decreased ILC following forest conversion to shaded-cocoa, it is also recommended that more studies be carried out on rainfall partitioning and its impact on ground water recharge as a way of establishing its influence on the availability of moisture for agriculture in these systems.

Salt marshes can sequester large amounts of carbon in sediments, but the relation between carbon storage and exportation remains poorly understood. Groundwater exchange can flush sediment carbon to surface waters and potentially reduce storage. In this study, we estimated groundwater fluxes and associated carbon fluxes using a radon (222Rn) mass balance and sediment carbon burial rates using lead (210Pb) in a pristine salt marsh (North Inlet, SC, USA). We used δ13C to trace carbon origins. We found that groundwater releases large amounts of carbon to the open ocean. These groundwater fluxes have the potential to export 7.2 ± 5.5 g m−2 of dissolved inorganic carbon (DIC), 0.2 ± 0.2 g m−2 of dissolved organic carbon (DOC) and 0.7 ± 0.5 g m−2 of carbon dioxide (CO2) per day. The fluxes exceed the average surface water CO2 emissions (0.6 ± 0.2 g m−2 day−1) and the average sediment carbon burial rates (0.17 ± 0.09 g m−2 day−1). The δ13C results suggest that groundwater carbon originated from salt marsh soils, while the sediment carbon source is derived from salt marsh vegetation. We propose that the impact of salt marshes in carbon cycling depends not only on their capacity to bury carbon in sediments, but also on their high potential to export carbon to the ocean via groundwater pathways.

A resumption of climate warming in maritime Antarctica, arising from continued greenhouse gas emissions to the atmosphere, is predicted to lead to further expansions of plant populations across the region, with consequent increases in nutrient inputs to soils. Here, we test the main and interactive effects of warming, applied with open top chambers (OTCs), and nutrient amendment with tryptic soy broth (TSB), an artificial growth substrate, on bacterial community composition and diversity using Illumina sequencing of 16S rRNA genes in soil from a field experiment in the southern maritime Antarctic. Substantial effects of TSB application on bacterial communities were identified after 49 months, including reduced diversity, altered phylogenetic community assembly processes, increased Proteobacteria-to-Acidobacteria ratios and significant divergence in community composition, notably increases in the relative abundances of the gram-positive genera Arthrobacter, Paeniglutamicibacter and Planococcus. Contrary to previous observations from other maritime Antarctic field warming experiments, we recorded no effects of warming with OTCs, or interactive effects of OTCs and TSB application, on bacterial community composition or diversity. Based on these findings, we conclude that further warming of the maritime Antarctic is unlikely to influence soil bacterial community composition or diversity directly, but that increased nutrient inputs arising from enhanced plant growth across the region may affect the composition of soil bacterial communities, with possible effects on ecosystem productivity.