Nitrogen storage in density and size fractions varied with tillage practices and cropping systems under residue return

Residue return can prevent or restore the degradation of cropland, meanwhile, additional N input from residue return inevitably result in the changes of soil nitrogen (N) pools. Our objectives were to evaluate these changes in a 16-year field experiment. The residue return experiment consisted of no-tillage (NT) and mouldboard plough (MP), combined with continuous maize (Zea mays L.) (MM) and maize-soybean (Glycine max Merr.) rotation (MS) cropping systems, that is, NTMM, NTMS, MPMM, MPMS; conventional tillage (removal of crop residue and deep plough) with continuous maize (CTMM) was included as a control. The soil was separated into density (LF, light fraction) and particle size (sand, silt and clay) fraction. In 0-5 cm and 5-10 cm layers, soil TN content in NT was higher than MP, whereas the opposite trend was observed in 10-20 cm. Thus, the stratification ratio of soil TN was greater under NT. Cropping system affected soil TN as MM > MS. Residue return increased soil N storage by 6.44%-24.85% in the plough layer. Taking CTMM as the baseline, NTMM and MPMM increased the N storage in all physical fractions, while the decrease of silt-N storage was observed in NTMS and MPMS. Under residue return, the distribution of N storage changes in LF and sand fraction was affected by tillage practice, and that in silt and clay fraction was affected by cropping system. In summary, NTMM is effective for soil N accumulation due to its highest N storage and all physical fractions of N storage was enhanced.

Recent Nitrogen Storage and Accumulation Rates in Mangrove Soils Exceed Historic Rates in the Urbanized San Juan Bay Estuary (Puerto Rico, United States)

Tropical mangrove forests have been described as “coastal kidneys,” promoting sediment deposition and filtering contaminants, including excess nutrients. Coastal areas throughout the world are experiencing increased human activities, resulting in altered geomorphology, hydrology, and nutrient inputs. To effectively manage and sustain coastal mangroves, it is important to understand nitrogen (N) storage and accumulation in systems where human activities are causing rapid changes in N inputs and cycling. We examined N storage and accumulation rates in recent (1970 – 2016) and historic (1930 – 1970) decades in the context of urbanization in the San Juan Bay Estuary (SJBE, Puerto Rico), using mangrove soil cores that were radiometrically dated. Local anthropogenic stressors can alter N storage rates in peri-urban mangrove systems either directly by increasing N soil fertility or indirectly by altering hydrology (e.g., dredging, filling, and canalization). Nitrogen accumulation rates were greater in recent decades than historic decades at Piñones Forest and Martin Peña East. Martin Peña East was characterized by high urbanization, and Piñones, by the least urbanization in the SJBE. The mangrove forest at Martin Peña East fringed a poorly drained canal and often received raw sewage inputs, with N accumulation rates ranging from 17.7 to 37.9 g m–2 y–1 in recent decades. The Piñones Forest was isolated and had low flushing, possibly exacerbated by river damming, with N accumulation rates ranging from 18.6 to 24.2 g m–2 y–1 in recent decades. Nearly all (96.3%) of the estuary-wide mangrove N (9.4 Mg ha–1) was stored in the soils with 7.1 Mg ha–1 sequestered during 1970–2017 (0–18 cm) and 2.3 Mg ha–1 during 1930–1970 (19–28 cm). Estuary-wide mangrove soil N accumulation rates were over twice as great in recent decades (0.18 ± 0.002 Mg ha–1y–1) than historically (0.08 ± 0.001 Mg ha–1y–1). Nitrogen accumulation rates in SJBE mangrove soils in recent times were twofold larger than the rate of human-consumed food N that is exported as wastewater (0.08 Mg ha–1 y–1), suggesting the potential for mangroves to sequester human-derived N. Conservation and effective management of mangrove forests and their surrounding watersheds in the Anthropocene are important for maintaining water quality in coastal communities throughout tropical regions.

Low N use Efficiency in A Long-Term N-Fertilized Cunninghamia Lanceolata Plantation Ecosystem in Subtropical China: A 14-Year Case Study

Background and aims Forests host among the most important N pools of all terrestrial ecosystems. Influences of N application on forest N cycle have received increasing concern, which is particularly problematic given the increasing atmospheric N deposition in recent decades. However, accurate assessments of N storage and recovery rates in forest ecosystems remain elusive. We selected Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) plantation ecosystem to explore how long-term N fertilization affected the N storage and recovery rate. Methods Plots in the field have been fertilized continuously for 14 years (2004–2017) with urea at rates of 0 (N0, control), 60 (N60, low-N), 120 (N120, medium-N) and 240 (N240, high-N) kg N hm− 2a− 1. Data collected in the field include N content and biomass on various plant organs (i.e., leaves, branches, stems, roots, and bark), understorey layer and litter in the ecosystem as well as soil N content and density at different depths (0–20, 20–40 and 40–60 cm). Key results The total N storage of ecosystem in the N-fertilized treatments was 1.1–1.4 times higher than that in the unfertilized treatment after 14 years of N fertilization. About 12.36% of the total ecosystem N was stored in vegetation (plant, litter, and understorey layer) and 87.64% was stored in soil (0–60 cm). N storage varied among ecosystem components and plant organs; and the plant organs, litter, and soil had higher N storage than that in understorey layer. Significantly higher Chinese fir N uptake was found in the medium-N (1.2 times) and high-N (1.4 times) treatments than that in the control. The N recovery rate of understorey layer in the N-fertilized treatments was negative, and less than that in the control. Conclusions Application of long-term N fertilizer to this stand led to a low N recovery rate (averagely 11.39%) while high loss of N (averagely 91.86%) which indicate low N use efficiency in the Chinese fir plantation ecosystem.

Cover Crop Residue Management for Effective Use of Mineralized Nitrogen in Greenhouse Tomato Production

Adequate residue management may enhance the benefits of cover crops on greenhouse tomato (Solanum lycopersicum L.) productivity, soil N pool, N cycling, and environmental quality. Regardless of management, cover crops may maintain or increase soil N storage at 10 cm depth compared with bare fallow. Cover crops may also enhance microbial biomass N, as a result, soil N availability may increase with cover crops, except rye (Secale cereale L.), more so with hairy vetch (Vicia villosa R.; HV) incorporation than HV mulch and the biculture of HV and rye. Residual inorganic N at surface soil may increase with cover crops, more so with HV and rye monocultures than the biculture. Tomato yield may increase more with the biculture than either HV incorporation or HV mulch because of an efficient residue-N use by tomatoes. The biculture may change the N release pattern from both cover crops: rye of the biculture may release more N than the monoculture, while HV may release a similar or more N in the late than in the early period of tomato growth. With adequate seeding HV/rye ratio (2/1), biculture may maintain or increase soil N storage, increase N cycling and tomato yield, and improve environmental quality.

Soil nitrogen sequestration in a long-term fertilizer experiment in central China

Aim of study: To evaluate the effects of a long-term manuring and fertilization experiment on the soil total N concentration and its storage and sequestration rates in the rice-wheat cropping system.Area of study: A rice-wheat rotation area in central China.Material and methods: A 35-yr long-term fertilizer experiment was conducted with 9 treatments: unfertilized (Control), N, P, and K fertilizers, manure (M) and M combined with N, P, and K fertilizers treatments. Soil total N input amount, total N concentration, total N storage amount and N sequestration rate in soil were calculated.Main results: The soil total N input amount, N concentration, N storage amount and N sequestration rate were significantly influenced by M and chemical fertilizers. In total, 0.017-0.021 g N/kg soil accumulated in the organic M plots, whereas only 0.005-0.007 g in chemical fertilizer alone plots. The highest soil total N storage amount was 6.09 t/hain the M alone plot, and the lowest value was 4.46 tN/ha in the N fertilizer alone plot. The highest N sequestration rate in soil was 0.061 t N/ha/yr in the high amount M plus NPK fertilizers plot, and the lowest value was 0.002 tN/ha/yr in the N fertilizer alone plot. A significant nonlinear regression relationship existed between the total N sequestration rate in soil and annual total N input amount. Moreover, the average soil total N concentration was significantly positively correlated with the average grain yield of crop and soil organic C concentration. The soil total N sequestration rate in M alone or M combined with inorganic fertilizer treatments were increased compared with inorganic fertilizer alone treatments.Research highlights: Considering crop yields and total N sequestration rate in soil, the use of manure combined with inorganic fertilizer should be recommended in the rice-wheat cropping system.

Nitrogen fertilizer regulates soil respiration by altering the organic carbon storage in root and topsoil in alpine meadow of the north-eastern Qinghai-Tibet Plateau

Soil respiration (Rs) plays a critical role in the global carbon (C) balance, especially in the context of globally increasing nitrogen (N) deposition. However, how N-addition influences C cycle remains unclear. Here, we applied seven levels of N application (0 (N0), 54 (N1), 90 (N2), 126 (N3), 144 (N4), 180 (N5) and 216 kg N ha−1 yr−1 (N6)) to quantify their impacts on Rs and its components (autotrophic respiration (Ra) and heterotrophic respiration (Rh)) and C and N storage in vegetation and soil in alpine meadow on the northeast margin of the Qinghai-Tibetan Plateau. We used a structural equation model (SEM) to explore the relative contributions of C and N storage, soil temperature and soil moisture and their direct and indirect pathways in regulating soil respiration. Our results revealed that the Rs, Ra and Rh, C and N storage in plant, root and soil (0–10 cm and 10–20 cm) all showed initial increases and then tended to decrease at the threshold level of 180 kg N ha−1 yr−1. The SEM results indicated that soil temperature had a greater impact on Rs than did volumetric soil moisture. Moreover, SEM also showed that C storage (in root, 0–10 and 10–20 cm soil layers) was the most important factor driving Rs. Furthermore, multiple linear regression model showed that the combined root C storage, 0–10 cm and 10–20 cm soil layer C storage explained 97.4–97.6% variations in Rs; explained 94.5–96% variations in Ra; and explained 96.3–98.1% in Rh. Therefore, the growing season soil respiration and its components can be well predicted by the organic C storage in root and topsoil in alpine meadow of the north-eastern Qinghai-Tibetan Plateau. Our study reveals the importance of topsoil and root C storage in driving growing season Rs in alpine meadow on the northeast margin of Qinghai-Tibetan Plateau.

Carbon and nitrogen in forest floor and mineral soil under four forest species in the Mediterranean region

The organic and mineral horizons of soils are of great importance in C and N storage in forest areas. However, knowledge of the effects of forest species on the stocks of these elements is still scarce, especially in Portugal. In order to contribute to this knowledge, a study was carried out in forest stands of <em>Pinus pinaster</em> Aiton (PP), <em>Pinus nigra</em> Arnold (PN), <em>Pseudotsuga menziesii</em> (PM) and <em>Castanea sativa</em> Miller (CS), installed in the 1950s in northern Portugal. Sampling areas with similar topography, lithology and climate were selected, in order to better identify hypothesized differences in C and N storage due to forest species effect. In each stand, 15 sites were selected randomly and the forest floor (organic layers) was collected in a 0.49 m² area. The layers H, L and F of the forest floor were identified and, for L and F, their components were separated in leaves, pine cones/chestnut husks and branches. At the same sites, soil samples were also collected at 0-10 and 10-20 cm depth. At these depths, undisturbed samples were also collected for bulk density determination. The concentrations of C and N were determined in forest floor and mineral components of the soil, and converted in mass per unit area. The quantity of C storage per unit area followed the sequence PN &gt; PM &gt; CS &gt; PP, while for N the sequence was CS &gt; PM &gt; PN &gt; PP, OM and PP keeping the same relative position in the sequence in both C and N concentrations. The PM and CS species store similar amounts of C and N, and about 90% of these elements is found in the upper 20 cm of the mineral soil. In PN and PP species, the contribution of forest floor to the storage of these elements is more expressive than in the other species, but lower than 30% in all cases.