Response of Gross Mineralization and Nitrification Rates to Banana Cultivation Sites Converted from Natural Forest in Subtropical China

Evaluations of gross mineralization (MNorg) and nitrification (ONH4) can be used to evaluate the supply capacity of inorganic N, which is crucial in determining appropriate N fertilizer application. However, the relevant research for banana plantations to date is limited. In this study, natural forest and banana plantations with different cultivation ages (3, 7, 10, and 22 y) were chosen in a subtropical region, and the 15N dilution technique was used to determine the gross MNorg and ONH4 rates. The objective was to evaluate the effect of the conversion of natural forests to banana plantations on inorganic N supply capacity (MNorg + ONH4) and other relevant factors. Compared to other natural forests in tropical and subtropical regions reported on by previous studies, the natural forest in this study was characterized by a relatively low MNorg rate and a high ONH4 rate in the soil, resulting in the presence of inorganic N dominated by nitrate. Compared to the natural forest, 3 y banana cultivation increased the MNorg and ONH4 rates and inorganic N availability in the soil, but these rates were significantly reduced with prolonged banana cultivation. Furthermore, the mean residence times of ammonium and nitrate were shorter in the 3 y than in the 7, 10, and 22 y banana plantations, indicating a reduced turnover of ammonium and nitrate in soil subjected to long-term banana cultivation. In addition, the conversion of natural forest to banana plantation reduced the soil organic carbon (SOC), total N and calcium concentrations, as well as water holding capacity (WHC), cation exchangeable capacity (CEC), and pH, more obviously in soils subjected to long-term banana cultivation. The MNorg and ONH4 rates were significantly and positively related to the SOC and TN concentrations, as well as the WHC and CEC, suggesting that the decline in soil quality after long-term banana cultivation could significantly inhibit MNorg and ONH4 rates, thus reducing inorganic N supply and turnover. Increasing the amount of soil organic matter may be an effective measure for stimulating N cycling for long-term banana cultivation.

The nitrification inhibitor dicyandiamide increases mineralization–immobilization turnover in slurry-amended grassland soil

SUMMARYNitrification inhibitors are used in agriculture for the purpose of decreasing nitrogen (N) losses, by limiting the microbially mediated oxidation of ammonium (NH4+) to nitrate (NO3−). Successful inhibition of nitrification has been shown in numerous studies, but the extent to which inhibitors affect other N transformations in soil is largely unknown. In the present study, cattle slurry was applied to microcosms of three different grassland soils, with or without the nitrification inhibitor dicyandiamide (DCD). A solution containing NH4+and NO3−, labelled with15N either on the NH4+or the NO3−part, was mixed with the slurry before application. Gross N transformation rates were estimated using a15N tracing model. In all three soils, DCD significantly inhibited gross autotrophic nitrification, by 79–90%. Gross mineralization of recalcitrant organic N increased significantly with DCD addition in two soils, whereas gross heterotrophic nitrification from the same pool decreased with DCD addition in two soils. Fungal to bacterial ratios were not significantly affected by DCD addition. Total gross mineralization and immobilization increased significantly across the three soils when DCD was used, which suggests that DCD can cause non-target effects on soil N mineralization–immobilization turnover.

Devenir de l'azote minéral dans une tourbière à Sphagnum fallax Klinggr. et Carex rostrata Stokes du Massif central (France)

The fate of mineral nitrogen (N) in an oligotrophic fen, located in Monts de Forez (Puy-de-Dôme department), was studied by 15N tracking of 0.5 g (nitrogen)·m-2·year-1 ammonitrate labelled on both ions (30% excess). Simulating atmospheric deposits, an important nutrient source in mires, N additions were made at two periods of the year, June and August, 1986, and the fate of the nitrogen was measured in October of the same year. Nitrogen recovery was measured in the living parts of Sphagnum fallax Klinggr., aerial and hypogeous parts of Carex rostrata Stokes, and in peat. Vegetation preferably absorbed nitrogen in June, as compared to August, the period during which nitrogen allocation to hypogeous phytomasses of Carex was the highest. As compared to Carex, Sphagnum has strong nitrogen assimilation and holding potentials. A nitrogen competition potential index or "nitrogen sink power" (PCN), calculated from recovery and stock data, showed that Sphagnum was more efficient at intercepting nitrogen in August, when rainfall is the lowest in the studied area. In peat, nitrogen recovery was strongest in August. Most of the microbial processes seem to occur in the first 10 cm. Gross nitrogen mineralization and immobilization, calculated from organic matter degradation, were estimated as 11.4 and 6.9 g·m-2·year-1, respectively. The gross mineralization estimation according to the method of Fried and Dean was 17.2 g·m-2·year-1. This overestimation might be attributed to an underestimation of the nitrogen derived from input in plants and an overestimation of nitrogen from peat.Key words: 15N, nitrogen balance, nitrogen uptake, competition, retention, turfigenous process.