Special Issue Editorial: Plant Nitrogen Assimilation and Metabolism

High N Storage but Low N Recovery After Long-Term N-Fertilization in a Subtropical Cunninghamia lanceolata Plantation Ecosystem: A 14-Year Case Study

Forests are among the most important N pools of all terrestrial ecosystems. Elevated atmospheric N deposition in recent decades has led to increased interest in the influences of N application on forest N cycles. However, accurate assessments of N storage in forest ecosystems remain elusive. We used a 14-year experiment of a Chinese fir [Cunninghamia lanceolata (Lamb.) Hook] plantation to explore how long-term N fertilization affected N storage and recovery rates. Our study plots were located in a field that had been continuously fertilized over 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 were collected that included N content and biomass in the understory, litter, and various plant organs (i.e., leaves, branches, stems, roots, and bark), as well as soil N content and density at different depths. Results showed that the total ecosystem N storage in the N-fertilized plots was 1.1-1.4 times higher than that in the control plots. About 12.36% of the total ecosystem N was stored in vegetation (plant organs, litter, and understory) and 87.64% was stored in soil (0-60 cm). Plant organs, litter, and soil had higher N storage than the understory layer. Significantly higher plant N uptake was found in the medium-N (1.2 times) and high-N (1.4 times) treatments relative to the control. The N recovery rate of the understory layer in the N-fertilized treatments was negative and less than that in the control. Application of long-term N fertilizer to this stand led to a low N recovery rate (average 11.39%) and high loss of N (average 91.86%), which indicate low N use efficiency in the Chinese fir plantation ecosystem. Our findings further clarify the distribution of N in an important terrestrial ecosystem and improve our understanding of regional N cycles.

Effects of nitrogen and phosphorus addition on growth and leaf nitrogen metabolism of alfalfa in alkaline soil in Yinchuan Plain of Hetao Basin

Alkaline soil is widely distributed in China. Its rational utilization is an effective measure to solve land shortage and improve the environment. Alfalfa is characterized by strong salt and alkali tolerance and high yield and protein content. Nitrogen (N) and phosphorus (P) are the main nutrients for plant growth, and N metabolism is one of the primary forms of plant metabolism, which plays a vital role in quality and yield formation. The exploration of the effect of N and P on N metabolism and alfalfa growth will provide a theoretical basis for scientific fertilization for alfalfa in the alkaline soil of the Yinchuan Plain of the Hetao Basin. Therefore, a 2-year experiment of N and P addition was conducted. Six treatments were set up with a randomized block design, including without N (WN), medium N (MN), high N (HN), without P (WP), medium P (MP), and high P (HP). It was found that the MN and MP treatments increased plant height, stem diameter, stem/leaf, dry/fresh, and dry matter of alfalfa. The HN and HP treatments inhibited alfalfa biomass formation. The MN and MP treatments increased key products and enzymes of leaf N metabolism of alfalfa and promoted activities of leaf nitrate reductase (NR), glutamine synthase (GS), glutamate synthase (GOGAT), glutamic-oxalacetic transaminase (GOT), and glutamic-pyruvate transaminase (GPT), and inhibited activities of leaf protease of alfalfa. The MN and MP treatments increased contents of leaf N, P, ammonium nitrogen (NH₄ ⁺-N), nitrate nitrogen (NO₃ ^--N), total chlorophyll, and protein and reduced leaf chlorophyll a/b and amino acid, results after HN and HP treatments were opposite. The correlation among leaf P, N, NO₃ ^--N, amino acid, and protein reached significant levels (P < 0.01). It is suggested that MN and MP treatments can improve the yield and quality of alfalfa by increasing key products and enzymes of N metabolism and can be adopted to promote alfalfa production in the alkaline soil of the Yinchuan Plain of the Hetao Basin.