Representation of phosphorus cycle in Joint UK Land Environment Simulator (vn5.5_JULES-CNP)

Most Land Surface Models (LSMs), the land components of Earth system models (ESMs), include representation of N limitation on ecosystem productivity. However only few of these models have incorporated phosphorus (P) cycling. In tropical ecosystems, this is likely to be particularly important as N tends to be abundant but the availability of rock-derived elements, such as P, can be very low. Thus, without a representation of P cycling, tropical forest response in areas such as Amazonia to rising atmospheric CO2 conditions remains highly uncertain. In this study, we introduced P dynamics and its interactions with the N and carbon (C) cycles into the Joint UK Land Environment Simulator (JULES). The new model (JULES-CNP) includes the representation of P stocks in vegetation and soil pools, as well as key processes controlling fluxes between these pools. We evaluate JULES-CNP at the Amazon nutrient fertilization experiment (AFEX), a low fertility site, representative of about 60 % of Amazon soils. We apply the model under ambient CO2 and elevated CO2. The model is able to reproduce the observed plant and soil P pools and fluxes under ambient CO2. We estimate P to limit net primary productivity (NPP) by 24 % under current CO2 and by 46 % under elevated CO2. Under elevated CO2, biomass in simulations accounting for CNP increase by 10 % relative to at contemporary CO2, although it is 5 % lower compared with CN and C-only simulations. Our results highlight the potential for high P limitation and therefore lower CO2 fertilization capacity in the Amazon forest with low fertility soils.

Soil Microbial Resource Limitations and Community Assembly Along a Camellia oleifera Plantation Chronosequence

Understanding soil microbial element limitation and its relation with the microbial community can help in elucidating the soil fertility status and improving nutrient management of planted forest ecosystems. The stand age of a planted forest determines the aboveground forest biomass and structure and underground microbial function and diversity. In this study, we investigated 30 plantations of Camellia oleifera distributed across the subtropical region of China that we classified into four stand ages (planted <9 years, 9–20 years, 21–60 years, and >60 years age). Enzymatic stoichiometry analysis showed that microbial metabolism in the forests was mainly limited by C and P. P limitation significantly decreased and C limitation slightly increased along the stand age gradient. The alpha diversity of the soil microbiota remained steady along stand age, while microbial communities gradually converged from scattered to clustered, which was accompanied by a decrease in network complexity. The soil bacterial community assembly shifted from stochastic to deterministic processes, which probably contributed to a decrease in soil pH along stand age. Our findings emphasize that the stand age regulated the soil microbial metabolism limitation and community assembly, which provides new insight into the improvement of C and P management in subtropical planted forest.

Responses of Microbial Nutrient Acquisition to Depth of Tillage and Incorporation of Straw in a Chinese Mollisol

Tillage and straw incorporation are important agricultural practices that can break the plow layer and improve Mollisol fertility. The effect of these practices on the limitation of resources for soil microorganisms, however, is unclear. We established a field experiment in 2018 and collection of soil samples in 2020 to study the acquisition of resources by microbes in a Mollisol region in northeastern China. Four treatments were studied: conventional tillage (CT), straw incorporation with conventional tillage (SCT), subsoil tillage (ST) and straw incorporation with subsoil tillage (SST). The limitation of resources for soil microorganisms was assessed using models of extracellular enzymatic stoichiometry. The soil microbes were generally colimited by C and P but not N. The degree of limitation, however, varied among the treatments. SCT and SST alleviated microbial P limitation in the 0–15 and 15–35 cm layers, respectively, but ST did not significantly affect P limitation relative to CT. Interestingly, N-resource contents were strongly correlated with indicators of C and P limitation. A random forest analysis found that the contents of available N and total dissolved N were the most important factors for microbial C and P limitation, respectively. Straw incorporation alleviated microbial P limitation but did not eliminate P limitation and deep tillage aggravate microbial C limitation. We suggest that N fertilization may be reduced due to the N-rich characteristics of the Mollisols in northeastern China.

Phosphorus for Sustainable Development Goal target of doubling smallholder productivity

Phosphorus (P) is an essential nutrient for life. In many tropical countries, P-fixing soils and very low historical P input limit uptake of P in crops and thus yields. This presents a serious obstacle for achieving the Sustainable Development Goal (SDG) target 2.3 of doubling productivity in smallholder farms. We calculated the geographic distribution of P limitation (1 – actual/potential P uptake) and the P input required to achieve this SDG target by 2030 in comparison to the Shared Socioeconomic Pathway (SSP2) scenario for five world regions where smallholder farms dominate. To achieve target 2.3, these regions require 39% more P application (126 Tg) between 2015 and 2030. While P limitation is most widespread in sub-Saharan Africa, it is the only region on track to achieving the doubling of productivity in the SSP2 scenario (increase by a factor of 1.8). Achieving the target requires a strong increase in P input, while protecting soils and waterways from excessive P runoff.

The Pattern of Leaf N:P Ratio Along Plantation Age: A Global Synthesis

Background Plant stoichiometry displayed flexible over age sequences in previous studies, but effect of stand age on stoichiometry showed large uncertainties for different plantations at the global scale. In this study, we want to obtain the general pattern of leaf N:P ratio for global plantations across age gradients through the compilation and analysis of published data from the individual studies. Results Stand age, together with life forms, and climatic variables strongly affected leaf N:P ratio. This result indicates that the stand age is an indispensable underlying mechanism on stoichiometry. Leaf N:P ratio increased with stand age for all plantations pooled together, revealing the existence of the general pattern in leaf stoichiometry for plantations across age gradients. Leaf N:P ratio exhibited no trend in evergreen trees, but an increasing trend was obtained in deciduous trees along age sequences. Meanwhile, leaf N:P ratio rose with stand age for plantations in humid subtropical regions, but it did not vary in dry temperate regions. Conclusions Our results reveal the effects of age on tree stoichiometry cannot be neglected. These age-related patterns of stoichiometry verified that global plantations changed from N limitation to P limitation with increasing stand age, particularly for deciduous plantations and in plantations in humid regions. The study highlights the importance of considering stand age when exploring nutrient patterns in plantations, which contributes to improving the plant stoichiometry theory and offers a guidance for the nutrient management of plantations at regional to global scales.

Precipitation Controls on Soil Biogeochemical and Microbial Community Composition in Rainfed Agricultural Systems in Tropical Drylands

The current and expected expansion of agriculture in the drylands of Mexico, together with the decrease in precipitation occurring in the country, likely affect ecosystem processes and will bring great challenges for the suitability of rainfed agriculture for smallholder farmers. Here, we assessed metrics of the soil C, N, and P cycles, as well as soil microbial diversity, under rainfed maize and common bean cropping in arid and semiarid regions of central Mexico. The soil enzymatic vector angles of cultivated plots in both regions were above 45°, suggesting P limitation for microbial growth and crop productivity. Although changes were not observed in the intensity of this P-limitation with aridity, we found a negative effect of drought increase on the concentration of soil organic C and total N, with consequences for the C, N, and P balance in soils. Increasing aridity leads to the homogenization of microbial diversity. Considering a scenario in which decreases in mean annual precipitation would uncouple the biogeochemical cycles and homogenize soil biodiversity, the ecological implications could be an increase in the vulnerability of agricultural ecosystems to drought, with negative consequences for the suitability of rainfed agriculture in the drylands of central Mexico.

Nutrient Alteration Drives the Impacts of Seawater Acidification on the Bloom-Forming Dinoflagellate Karenia mikimotoi

Seawater acidification and nutrient alteration are two dominant environmental factors in coastal environments that influence the dynamics and succession of marine microalgae. However, the impacts of their combination have seldom been recorded. A simulated experimental system was set up to mimic the effects of elevated acidification on a bloom-forming dinoflagellate, Karenia mikimotoi, exposed to different nutrient conditions, and the possible mechanism was discussed. The results showed that acidification at different pH levels of 7.6 or 7.4 significantly influenced microalgal growth (p<0.05) compared with the control at pH 8.0. Mitochondria, the key sites of aerobic respiration and energy production, were impaired in a pH-dependent manner, and a simultaneous alteration of reactive oxygen species (ROS) production occurred. Cytochrome c oxidase (COX) and citrate synthase (CS), two mitochondrial metabolism-related enzymes, were actively induced with acidification exposure, suggesting the involvement of the mitochondrial pathway in coping with acidification. Moreover, different nutrient statuses indicated by various N:P ratios of 7:1 (N limitation) and 52:1 (P limitation) dramatically altered the impacts of acidification compared with those exposed to an N:P ratio of 17:1 (control), microalgal growth at pH 7.4 was obviously accelerated with the elevation of the nutrient ratio compared to that at pH 8.1 (p<0.05), and nutrient limitations seemed beneficial for growth in acidifying conditions. The production of alkaline phosphatase (AP) and acid phosphatase (AcP), an effective index indicating the microalgal growth status, significantly increased at the same time (p<0.05), which further supported this speculation. However, nitrate reductase (NR) was slightly inhibited. Hemolytic toxin production showed an obvious increase as the N:P ratio increased when exposed to acidification. Taken together, mitochondrial metabolism was suspected to be involved in the process of coping with acidification, and nutrient alterations, especially P limitation, could effectively alleviate the negative impacts induced by acidification. The obtained results might be a possible explanation for the competitive fitness of K. mikimotoi during bloom development.

Bioprocess Optimization for the Production of Arthrospira (Spirulina) platensis Biomass Enriched in the Enzyme Alkaline Phosphatase

The enzyme alkaline phosphatase (ALP) is gaining interest because it exerts bioactive properties and may be a potentially important therapeutic agent for many disorders and diseases. Microalgae are considered an important novel source for the production of diverse bio-compounds and are gaining momentum as functional foods/feeds supplements. So far, studies for the production of ALP are limited to mammalian and partly to some heterotrophic microbial sources after its extraction and/or purification. Methods: Arthrospira was cultivated under P-limitation bioprocess and the effect of the P-limitation degree on the ALP enrichment was studied. The aim of this work was to optimize the cultivation of the edible and generally-recognized-as-safe (GRAS) cyanobacterium Arthrospira platensis for the production of single-cell (SC) biomass enriched in ALP as a potential novel functional diet supplement. Results: The results revealed that the relationship between intracellular-P and single-cell alkaline phosphatase (SC-ALP) activity was inverse; SC-ALP activity was the highest (around 50 U g−1) when intracellular-P was the lowest possible (around 1.7 mg-P g−1) and decreased gradually as P availability increased reaching around 0.5 U g−1 in the control cultures. Under the strongest P-limited conditions, a more than 100-fold increase in SC-ALP activity was obtained; however, protein content of A. platensis decreased significantly (around 22–23% from 58%). Under a moderate P-limitation degree (at intracellular-P of 3.6 mg-P g−1), there was a relatively high SC-ALP activity (>28 U g−1) while simultaneously, a relative high protein content (46%) was attained, which reflects the possibility to produce A. platensis enriched in ALP retaining though its nutritional value as a protein rich biomass source. The paper presents also results on how several parameters of the ALP activity assay, such as pH, temperature etc., and post-harvest treatment (hydrothermal treatment and biomass drying), influence the SC-ALP activity.

Topography modulates effects of nitrogen deposition on microbial resource limitation in a nitrogen-saturated subtropical forest

Background Nitrogen (N) saturation theory proposes that an ecosystem might switch from N limitation to carbon (C), phosphorus (P), or other nutrient limitations if it receives continuous N input. Yet, after N limitation is removed, which nutrient is the most limited and whether topography modulates such change is rarely tested at a microbial level. Here, we conducted a two-year N addition experiment under two different topography positions (i.e. a slope and a valley) in a N-saturated subtropical forest. Soil enzyme activity was measured, and ecoenzymatic stoichiometry indexes were calculated as indicators of microbial resource limitation. Results In the valley, two-year N addition changed the activity of all studied enzymes to various degrees. As a result, microbial C limitation was aggravated in the valley, and consequently microbial decomposition of soil labile organic C increased, but microbial P limitation was alleviated due to the stoichiometry balance. On the slope, however, N addition did not significantly change the activity of the studied enzymes, and did not alter the status of microbial resource limitation. Conclusions These results indicate that C is a more limited element for microbial growth than P after removing N limitation, but we also highlight that topography can regulate the effect of N deposition on soil microbial resource limitation in subtropical forests. These findings provide useful supplements to the N saturation theory.