Nitrapyrin affects the abundance of ammonia oxidizers rather than community structure in a yellow clay paddy soil

Purpose

Yellow clay paddy soil (Oxisols) is a low-yield soil with low nitrogen use efficiency (NUE) in southern China. The nitrification inhibitor nitrapyrin (2-chloro-6- (tricholoromethyl)-pyridine, CP) has been applied to improve NUE and reduce environmental pollution in paddy soil. However, the effects of nitrapyrin combined with nitrogen fertilizers on ammonia oxidizers in yellow clay paddy soil have not been examined.

Materials and methods

A randomized complete block design was set with three treatments: (1) without nitrogen fertilizer (CK), (2) common prilled urea (PU), and (3) prilled urea with nitrapyrin (NPU). Soil samples were collected from three treatments where CK, PU, and NPU had been repeatedly applied over 5 years. Soil samples were analyzed by quantitative PCR and 454 high-throughput pyrosequencing of the amoA gene to investigate the influence of nitrapyrin combined with nitrogen on the abundance and community structure of ammonia oxidizers in yellow clay paddy soil.

Results and discussion

The potential nitrification rate (PNR) of the soil was significantly correlated with the abundances of both ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). Application of urea significantly stimulated AOA and AOB growth, whereas nitrapyrin exhibited inhibitory effects on AOA. Phylogenetic analysis showed that the most dominant operational taxonomic units (OTUs) of AOA and AOB were affiliated with the Nitrosotalea cluster and Nitrosospira cluster 12, respectively. AOA and AOB community structures were not altered by urea and nitrapyrin application.

Conclusions

Nitrogen fertilization stimulated nitrification and increased the population sizes of AOA and AOB. Nitrapyrin affected the abundance, but not community structure of ammonia oxidizers in yellow clay soil. Our results suggested that nitrapyrin improving NUE and inhibiting PNR was attributable to the inhibition of AOA growth.

     

Main environmental drivers of abundance, diversity and community structure of comammox Nitrospira in paddy soils

The recently discovered complete ammonia oxidizers comammox Nitrospira contain clades A and B that can establish an independent one-step nitrification process; however, little is known about their environmental drivers or habitat distributions in agricultural soils. Previous studies on comammox Nitrospira in paddy soils have mainly focused on small-scale samples, and there is a lack of multisite research on comammox Nitrospira in paddy soils. In this study, we conducted a survey of 36 paddy soils to understand the community structure, abundance, and diversity of comammox Nitrospira and the degree to which they are affected by environmental factors at a large scale. Comammox Nitrospira were found to be widely distributed among the paddy soils. The abundance of comammox Nitrospira clade A was mostly lower than that of clade B, whereas its diversity was mostly higher than that of clade B. Correlation analysis showed that multiple factors affected (P < 0.05) the abundance of comammox Nitrospira, including soil pH, organic matter, total carbon, and total nitrogen, latitude, mean annual temperature, and mean annual precipitation. Moreover, there was a clear relationship between the comammox Nitrospira community and habitat, indicating that some amplicon sequence variants (ASVs) had a unique dominant position in specific habitats. Phylogenetic analysis showed that the ASVs of comammox Nitrospira clade A clustered with the known sequences in the paddy soils and were significantly different from the known sequences in other habitats, which may be related to the unique paddy field habitat. In contrast, comammox Nitrospira clade B showed no clear habitat dependence. These results support the wide distribution and high abundance of comammox Nitrospira in paddy soils and provide novel insights into nitrogen cycling and nutrient management in agricultural ecosystems.     

Interactions between phosphorus availability and an AM fungus (Glomus intraradices) and their effects on soil microbial respiration, biomass and enzyme activities in a calcareous soil

The interactions between soil P availability and mycorrhizal fungi could potentially impact the activity of soil microorganisms and enzymes involved in nutrient turnover and cycling, and subsequent plant growth. However, much remains to be known of the possible interactions among phosphorus availability and mycorrhizal fungi in the rhizosphere of berseem clover (Trifolium alexandrinum L.) grown in calcareous soils deficient in available P. The primary purpose of this study was to look at the interaction between P availability and an arbuscular mycorrhizal (AM) fungus (Glomus intraradices) on the growth of berseem clover and on soil microbial activity associated with plant growth. Berseem clover was grown in P unfertilized soil (−P) and P fertilized soil (+P), inoculated (+M) and non-inoculated (−M) with the mycorrhizal fungus for 70 days under greenhouse conditions. We found an increased biomass production of shoot and root for AM fungus-inoculated berseem relative to uninoculated berseem grown at low P levels. AM fungus inoculation led to an improvement of P and N uptake. Soil respiration (SR) responded positively to P addition, but negatively to AM fungus inoculation, suggesting that P limitation may be responsible for stimulating effects on microbial activity by P fertilization. Results showed decreases in microbial respiration and biomass C in mycorrhizal treatments, implying that reduced availability of C may account for the suppressive effects of AM fungus inoculation on microbial activity. However, both AM fungus inoculation and P fertilization affected neither substrate-induced respiration (SIR) nor microbial metabolic quotients (qCO₂). So, both P and C availability may concurrently limit the microbial activity in these calcareous P-fixing soils. On the contrary, the activities of alkaline phosphatase (ALP) and acid phosphatase (ACP) enzymes responded negatively to P addition, but positively to AM fungus inoculation, indicating that AM fungus may only contribute to plant P nutrition without a significant contribution from the total microbial activity in the rhizosphere. Therefore, the contrasting effects of P and AM fungus on the soil microbial activity and biomass C and enzymes may have a positive or negative feedback to C dynamics and decomposition, and subsequently to nutrient cycling in these calcareous soils. In conclusion, soil microbial activity depended on the addition of P and/or the presence of AM fungus, which could affect either P or C availability.     

Relationships between C and N availability, substrate age, and natural abundance C and N signatures of soil microbial biomass in a semiarid climate

Soil microbial organisms are central to carbon (C) and nitrogen (N) transformations in soils, yet not much is known about the stable isotope composition of these essential regulators of element cycles. We investigated the relationship between C and N availability and stable C and N isotope composition of soil microbial biomass across a three million year old semiarid substrate age gradient in northern Arizona. The δ¹⁵N of soil microbial biomass was on average 7.2‰ higher than that of soil total N for all substrate ages and 1.6‰ higher than that of extractable N, but not significantly different for the youngest and oldest sites. Microbial ¹⁵N enrichment relative to soil extractable and total N was low at the youngest site, increased to a maximum after 55,000 years, and then decreased slightly with age. The degree of ¹⁵N enrichment of microbial biomass correlated negatively with the C:N mass ratio of the soil extractable pool. The δ¹³C signature of soil microbial biomass was 1.4‰ and 4.6‰ enriched relative to that of soil total and extractable pools respectively and showed significant differences between sites. However, microbial ¹³C enrichment was unrelated to measures of C and N availability. Our results confirm that ¹⁵N, but not ¹³C enrichment of soil microbial biomass reflects changes in C and N availability and N processing during long-term ecosystem development.     

Spatial dependency of soil nutrient availability and microbial properties in a mixed forest of Tsuga heterophylla and Pseudotsuga menziesii, in coastal British Columbia, Canada

Spatial variations in nutrient concentrations and turnover may contribute to variations in productivity within forest ecosystems and be responsible for creating and maintaining diversity of plant species. The aim of this study was to relate spatial patterns in soil nutrient availability and microbial properties in the forest floor and mineral soil in order to explore the controls on variations in nutrient availability in the two horizons. Microbial C, N and P, extractable N and P, and plant-available nutrients as estimated by plant root simulator probes were measured. We then used geostatistical techniques to determine the scale of spatial autocorrelation in the forest floor and mineral soil. Several of the measured variables were spatially autocorrelated at a scale of tens to hundreds of meters. Although more variables were autocorrelated in the mineral soil than in the forest floor, spatial patterns in gravimetric moisture content and nutrient concentrations in the two horizons generally overlapped. The spatial patterns were probably caused by differences in topography, except for ammonium whose shorter range of spatial autocorrelation may reflect variations in nitrogen content of the canopy.     

Behavior of iodine in a forest plot,an upland field and a paddy field in the upland area of Tsukuba,Japan: Seasonal variations in iodine concentration in precipitation and soil water and estimation of the annual iodine accumulative amount in soil horizons

Abstract

In the course of a series of studies conducted to investigate the long-term behavior of ¹²⁹I (which has a half-life of 16 million years) in the environment, seasonal variation in the concentration of stable iodine (¹²⁷I) in precipitation and soil water to a depth of 2.5 m in a forest plot, an upland field and a paddy field in the upland area of Tsukuba, Japan, were determined. Iodine concentration in precipitation tended to increase during the summer (high air temperature) season and low-rainfall period, and a positive high correlation was observed between annual rainfall and the annual amount of iodine supplied by precipitation. No seasonal variations in iodine concentration in soil water were observed at any depth in the forest plot and upland field unlike at shallow depths (0.2 and 0.5 m) in the paddy field. In the paddy field, from the beginning of summer irrigation, under flooding conditions, iodine concentration in soil water at shallow depths (0.2 and 0.5 m) continuously increased, and immediately before mid-summer (intermittent) drainage and drainage, the maximum iodine concentration (approximately 50 µg L^?1) and lowest E_h values (approximately ?150 to ?200 mV) were recorded. These high iodine concentration levels and low E_h values were ascribed to high air temperature (approximately > 25°C on average every 10 days) and the continuation of the groundwater level above the ground surface. As for the temporary winter irrigation period (mean daily air temperature 2?4°C), the iodine concentration was low (1.7–3.7 µg L^?1) at all depths, as was the case in the non-irrigation period. After mid-summer drainage, and drainage, the iodine concentration in soil water at depths of 0.2 and 0.5 m decreased drastically as the groundwater level decreased. The mean annual amount of iodine accumulated in the surface soil horizons (0–0.67 m) in the forest plot was estimated to be approximately 2.9 mg m^?2 (7.5 µg kg^?1 dry soil), which coincided with the mean annual amount of iodine supplied to the earth surface by precipitation. A mildly oxidative subsurface 2Bw horizon (0.60–0.89 m) in the paddy field was estimated to illuviate approximately 3.1 mg m^?2 (20 µg kg^?1 dry soil) of iodine annually by retaining iodine in the soil water percolated to this horizon.