Xin Chen

The dynamics of phosphorus (P) in litter–soil systems during litter decomposition across a plantation chronosequence remain to be underinvestigated, especially in terms of the nutrient cycle in plantations. In this study, the P dynamics in a litter–soil system of larch (Larix kaempferi) plantations at three stand ages (10, 25, and 50 years old) were examined through a 4-year in situ decomposition experiment (experiment 1) and a 360-day indoor incubation experiment (experiment 2). The aim of experiment 1 and experiment 2 is to determine the P dynamics in litter and soil, respectively. The results in experiment 1 suggested that litter mass retained 34.1%–42.5% of the initial mass after a 4-year decomposition period, and the turnover time (t_0.95) of the decomposition was 11.3, 13.9, and 11.8 years for 10-, 25- and 50-year-old stand larch plantations, respectively. Litter exhibited a net P decrease during the first 180 days, followed by a phase of a net P increase. The lowest P accumulation rate was found in the 25-year-old stand during the P immobilization stage. This immobilization phase was followed by a slow litter P decrease. Highly correlated relations were found between the litter decomposition rate and the initial litter N concentration and C/N, whereas the P accumulation rate was noticeably correlated with the initial litter P and C/P. The results in experiment 2 showed that litter addition promoted the accumulation of the highly labile P (resin P, NaHCO₃-P_i, and NaHCO₃-P_o), as well as moderately labile P_i (NaOH-P_i) in the soil. The findings obtained suggest that soil microbial biomass P and acid phosphatase activity were the primary factors driving the activation of soil P during litter decomposition. These findings would be beneficial to the systematic understanding of the nutrient cycle in plant–soil systems and litter management during the development of larch plantations.

Since the 1950s, nearly 80% of wetlands in the Sanjiang Plain have been converted into paddy fields. The conversion might affect the solubility and mobility of soil iron, influencing the export of iron into the Amur River and the primary production of the Okhotsk Sea. However, information regarding long‐term studies of the spatiotemporal dynamics of soil iron after cultivation is limited. In this study, six regions, including 18 plots in the Sanjiang Plain, were selected as sampling sites covering natural wetlands and paddy fields with planting ages of 2, 5, 11, 18, and 25 years after conversion from the wetland. Samples were collected at six different depths (0–10, 10–20, 20–40, 40–60, 60–90, and 90–120 cm) analyzed for water‐soluble ferrous iron (Fe[II]), water‐soluble iron (Fe_w), complex iron (Fe_p), amorphous iron oxides (Fe_o), free iron oxides (Fe_d), and total iron (Fe_t) and six soil physicochemical characteristics. Two years after the conversion of wetlands to rice fields led to an immediate decrease in Fe(II), Fe_w, Fe_p/Fe_d, and Fe_o/Fe_d, while the Fe_p and Fe_o contents decreased at 5 years. Both the concentrations and stocks of soil Fe_t increased gradually during the first 18 years. The findings in the Sanjiang Plain suggest that the function of wetlands after conversion as a source of iron might decrease with increasing time, with potential ecological effects on the neighboring marine environment. Recently initiated wetland restoration would protect the land ecosystems in the Sanjiang Plain and promote the future sustainability of the Amur Basin.

Nitrate excess is common in greenhouse soils, imposing environmental risks and degrading vegetable quality. In this study, the effectiveness of adding sucrose as available carbon through irrigation to cut nitrate excess in lettuce-planted soil was investigated under impacts of soil texture and irrigation type. In the pot experiment using two loam soils of same origin with different clay to sand ratios (50.2% and 39.8%) and nitrate excess (116.1 and 417.7 mg/kg N), three-time sucrose addition through flood irrigation was more effective in lowering net formation of nitrate-based inorganic N and increasing lettuce yield in the soil with the higher clay to sand ratio, and sucrose addition at 150–450 mg/kg reduced nitrate accumulation and leaching, and nitrate content of lettuce at harvest by 62.5–89.6%, 19.3–36.1%, and 11.4–76.0%, respectively. In the micro-plot field experiment with two-time sucrose addition at 0.6–1.2 g/L through furrow irrigation (42 mm) into two other soils of same origin with different clay to sand ratios (56.9%, 48.4%), nitrate accumulation at 0–30-cm depth at the prone-leaching furrow location at harvest decreased by 30.9–36.0% under the higher clay to sand ratio but increased by about 35% under the lower clay to sand ratio. The nitrate content and yield of ridge-planted lettuce was less affected in either soil. Hence, carbon addition rate, irrigation type, and clay to sand ratio all affected the effect of available carbon addition on nitrate accumulation in vegetable-planted soil, and their joint impacts need better quantification for cutting nitrate excess in soil and improving vegetable quality and even yield.

Summary: Iron is an important factor in algal blooms because it is involved in cyanobacterial pigment biosynthesis and therefore has the ability to influence the pigment status of algal cells. This role in pigment biosynthesis offers the opportunity for rapid monitoring of iron availability to cyanobacteria through spectral reflectance characterization. In the present study, the freshwater cyanobacterium Microcystis viridis was cultured with different levels of iron. Cell density, cellular content of iron and photosynthetic pigments, and spectral reflectivity of M. viridis were determined daily during the course of the culture experiment. The results showed that at the lowest iron concentration (0.01 μM) the growth of M. viridis was seriously limited, and the maximal cell density was only approximately 6.4% of the density observed with an iron concentration of 18 μM. Iron availability dramatically affected chlorophyll a, carotenoid and phycocyanin content, with the greatest impact on chlorophyll a. The iron‐induced changes in content and ratios of pigments were detectable through spectral reflectance. Eleven spectral indices previously developed for the estimation of concentrations and/or ratios of pigments and a newly proposed chlorophyll a/phycocyanin index were found to be suitable for generating sensitive regression models between cellular iron content and spectral parameters. The comprehensive application of key sensitive spectral indices and regression equations should help to support monitoring and diagnosis of iron availability to cyanobacteria via remote sensing.