Nutrient resorption processes in the plants infected by pathogen remain poorly understood. Huanglongbing (HLB) is a destructive disease of citrus. HLB-pathogen ‘Candidatus Liberibacter asiaticus’ grows specifically in the phloem of hosts and may cause problems in the plant vascular system after infection. Therefore, it brings a great concern about the phloem nutrient transport and nutrient intra-cycling in HLB-affected plants. We investigated the effects of ‘Ca. L. asiaticus’ infection on nitrogen (N) and phosphorus (P) concentrations and resorption in different citrus species (i.e. Citrus reticulata, Citrus limon and Citrus maxima). HLB-pathogen infection had distinctive impacts on nutrient resorption in different species. P resorption efficiency substantially decreased in infected C. reticulata plants relative to the healthy plants in summer, which may account for the marked decrease in the average fruit yield. P resorption was more efficient in infected C. limon plants than in the healthy plants. However, for C. maxima plants, HLB had no significant effects on N:P ratio in live leaves and resorption efficiency as well as on fruit yield. Keeping efficient internal nutrient cycling can be a strategy of citrus species being tolerant to HLB.

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Atmospheric nitrogen (N) deposition, generally, has been simulated through a single or relatively few N applications per year for its ecological effect study. Despite the importance of timing in ecosystem processes, ecological experiments with more realistic N addition frequencies are rare. We employed a novel design with typical twice (2X) versus atypical monthly (12X) N applications per year to explore effects of N addition frequency on above‐ and below‐ground biodiversity and function. Each year, several response variables from either below‐ground or above‐ground growth, N status and cycling, or plant and bacterial diversity differed as a result of N addition frequency. BNPP showed a large frequency effect in the relatively moist year but not in the dry year. Nitrogen addition decreased root growth in the monthly relative to the biannual applications, which could be highly consequential for predicting changes in global carbon and nitrogen cycling. Simulated N deposition tended to perturb biodiversity, but it is noteworthy that 12X applications that spread N deposition more evenly through a year have much less negative impacts on plant and bacterial diversities than 2X amendments per year. Soil N mineralization rate in year 6 was much lower when N additions were monthly compared with a biannual amendment, especially when simulated N deposition was high. We have established that amendment frequency matters for understanding ecosystem response to N deposition. Experiments that more closely mimic the anthropogenic process of N deposition are needed to best assess ecosystem and potential global biogeochemical changes. A free Plain Language Summary can be found within the Supporting Information of this article.

The environment-friendly disposal approaches for sewage sludge remain a challenge worldwide, and agricultural application of the treated sewage sludge, i.e. biosolids, as soil conditioner and nutrient source for plants, is still one of the most promising options for dealing with the waste. In this study, we compared the effects of long-term applications of biosolids and mineral fertilizer on soil microbial community using phospholipid fatty acid profiles in an agricultural field. The microbes predominant in the mineral fertilizer soil remained the most abundant in the biosolids-amended soil. Bacterial and fungal communities presented downtrends in both absolute and relative amounts; however, percentage of Gram-positive bacteria remained unchanged. Soil pH displayed significant negative correlations with microbial communities and explained 64%, 56% and 44% of the variances in microbial biomass, bacteria and fungi, respectively, while soil NO 3-N was positively correlated with microbial biomass, bacteria and fungi (r 2 = 0.53, 0.51 and 0.54, respectively).

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Studies exploring ecosystem vulnerability to nitrogen (N) enrichment have mostly focused on aboveground components of ecosystems. However, the sensitivity of the belowground ecosystem to increasing N deposition remains unclear. We estimated responses of belowground net primary productivity (BNPP), soil microbial biomass N (MBN), N mineralization, nitrification, and 16S rRNA gene based bacterial diversity to elevated N inputs. The study was based on a long‐term N deposition experiment with monthly N applications at nine rates ranging from 0 to 50 g N m−2 yr−1 in a temperate grassland. BNPP, MBN, and microbial diversity showed non‐linearities across the N gradient. In both post‐hoc test and regression tree model, the significant decrease of BNPP relative to the control started at the 10 g N m−2 yr−1 rate (the 53% decrease), whereas in Bayesian regression model, the decrease started at the 5 g N m−2 yr−1 rate (the 39% decrease) in year 6. We therefore estimated a critical load range of 5–10 g N m−2 yr−1 for BNPP. Regression tree model, Bayesian regression, and post‐hoc test consistently suggested that the detrimental effects on MBN might occur above ∼10 g N m−2 yr−1 addition rate. Bacterial diversity and the relative abundance of dominant phyla declined when N addition rate exceeded 5 or 10 g N m−2 yr−1. The impacts of N deposition on the root‐microbe system strongly depended on the interannual fluctuation in precipitation. The responses of the sensitive belowground indicators are vital to help minimize the detrimental impacts of anthropogenic N inputs.

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