Effects of conversion of natural forest to plantations on the abundance of nitrite reducing genes in soil aggregates in subtropical forest region.

Large proportion of natural forest has been transformed into plantations in subtropical regions, with consequences on forest ecosystem structure and function. In order to understand the responses of two nitrite reducing genes (nirK and nirS) in N2O production to forest conversion, we collected soil samples from Castanopsis carlesii natural forest, Cunninghamia lanceolata plantation and Pinus massoniana plantation and examined the abundance of nirK and nirS genes in soils and aggregates. Results showed that forest conversion increased soil pH, while decreased soil ammonium content. Forest conversion did not influence the mass proportion of soil aggregates. The abundance of nirK and nirS genes varied in aggregates with different particle sizes. The abundance of nirK and nirS genes was the highest in small macraoaggregates and the lowest in the silt-clay particles. Moreover, the abundance of nirK was significantly higher than that of nirS in soils of all forest types, indicating that nirK dominated in the acidic forest soils. Conversion of natural forest to plantations significantly increased the abundance of nirK and nirS genes in the bulk soil and aggregates, indicating that forest conversion would be beneficial for the growth of microorganisms bearing nirK and nirS genes, which might be associated with the increases of soil pH. Taken together, conversion of natural forest to C. lanceolata plantation or P. massoniana plantation significantly increased the abundance of nirK and nirS in soils and aggregates, but did not affect the mass proportions of aggregates.

[Responses of soil ammonia-oxidizing microorganisms to simulated nitrogen deposition in a natural Castanopsis carlesii forest]. / äºšçƒ­å¸¦ç±³æ§ å¤©ç„¶æž—åœŸå£¤æ°¨æ°§åŒ–å¾®ç”Ÿç‰©å¯¹æ¨¡æ‹Ÿæ°®æ²‰é™çš„å“åº”.

Subtropical region of China is one of the global hotspots receiving nitrogen deposition. Nitrogen deposition could affect the abundance and community structure of ammonia oxidizers including ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA) and complete ammonia oxidizer (comammox Nitrospira), with consequences on soil nutrient cycling that are driven by microorganisms. There is limited understanding for the newly discovered comammox Nitrospira in the subtropical forest soils. Here, we investigated the effect of simulated N deposition on abundances of soil ammonia oxidizers in the Castanopsis fargesii Nature Reserve in Xinkou Town, Sanming City, Fujian Province, China. Soil samples were collected from the field plots which received long-term nitrogen deposition with different dosages, including: CK, no additional treatment; LN, low nitrogen deposition treatment, dosage of 40 kg N·hm-2·a-1; and HN, high nitrogen deposition treatment, dosage of 80 kg N·hm-2·a-1. After 8-year treatment, simulated N deposition decreased soil pH and organic matter content, and increased nitrate content. We failed to amplify the amoA gene of AOB in the tested soils. High nitrogen deposition increased the abundance of AOA, but did not affect the abundance of comammox Nitrospira clade A and clade B. The ratio of comammox Nitrospira to AOA decreased with N addition, indicating that N addition weakened the role of comammox Nitrospira in nitrification in the subtropical forest soils. However, there were strong non-specific amplifications for both comammox Nitrospira clades A and B, highlighting the demand for the development of high coverage and specificity primers for comammox Nitrospira investigations in the future. The abundance of comammox Nitrospira clade A was positively correlated with total nitrogen (TN) and NH4+ concentration, while that of clade B was positively associated with soil organic carbon (SOC), TN and NH4+ Concentration. Overall, our findings demonstrated that simulated N deposition increased the relative importance of AOA in nitrification in the natural Castanopsis carlesii forest soil. These findings could provide theoretical support in coping with global change and N deposition in these regions.

[Long-term effects of imbalanced fertilization with lime and gypsum additions on denitrifying functional genes of an Ultisol at Yingtan, Jiangxi, China]. / é•¿æœŸç¼ºç´ æ–½è‚¥åŠçŸ³ç°çŸ³è†æ–½ç”¨å¯¹æ±Ÿè¥¿é¹°æ½­çº¢å£¤åç¡åŒ–å¾®ç”Ÿç‰©åŠŸèƒ½åŸºå› ä¸°åº¦çš„å½±å“.

The abundance of denitrifying functional genes plays a key role in driving the soil nitrous oxide (N2O) emission potential. Nitrite reductase genes (nirK and nirS) and nitrous oxide reductase genes (nosZ I and nosZ II) are the dominant denitrifying funtional genes. In this study, real-time quantitative PCR was conducted to evaluate the effects of 32-year imbalanced fertilization and lime and gypsum additions on the abundances of nirK, nirS, nosZ I and nosZ II genes in an Ultisol at Yingtan, Jiangxi Province. We further explored the underlying driving factors. The results showed that, compared with the balanced fertilization treatment, fertilization without phosphorus (P) signifi-cantly decreased the abundances of nirK, nirS, nosZ I and nosZ II genes. Fertilization without nitrogen (N) significantly reduced the abundances of nirK, nosZ I and nosZ II, but did not affect the abundance of nirS. Fertilization without potassium (K) did not affect the abundances of all denitri-fying functional genes. Results of stepwise regression analysis and random forest analysis showed that soil pH was a key environmental factor affecting the abundances of nosZ I and nosZ II. The application of lime or lime + gypsum significantly increased soil pH, which subsequently increased the abundances of nosZ II and nosZ II/nosZ I by 150%-231% and 127%-155%, respectively. Our results suggested that application of lime or lime + gypsum favored nosZ II more than nosZ I in upland Ultisols, which might enhance the relative importance of nosZ II in N2O reduction. Overall, fertilization without P would reduce denitrifying gene abundances, while the application of lime or lime + gypsum enriched nosZ II and increased ratio of nosZ II/nosZ I, which might be beneficial for reducing N2O emission potential in the Ultisols.

[Responses of denitrifying functional gene abundance to long-term fertilization regimes in an upland Ultisol]. / æ—±åœ°çº¢å£¤åç¡åŒ–åŠŸèƒ½åŸºå› ä¸°åº¦å¯¹é•¿æœŸæ–½è‚¥çš„å“åº”.

Fertilization affects soil nitrogen cycling and nitrous oxide (N2O) emissions, which are mainly driven by microbes. A 32-year field experiment was conducted to investigate the effects of chemical fertilizers and their combination with organic materials on the abundance of denitrifying functional genes (nirS, nirK, nosZ I and nosZ II) in Ultisol. The treatments comprised no fertilizer (CK), chemical fertilizer, chemical fertilizer+peanut straw, chemical fertilizer+rice straw, chemical fertilizer+radish and chemical fertilizer+pig manure. Compared with the single chemical fertilizer treatment, soil pH and organic carbon content increased in the chemical fertilizer plus organic material treatments, with chemical fertilizer+pig manure having the strongest effect. Long-term fertilization did not affect the abundance of nirK gene, but significantly altered the nirS gene abundance. Compared to CK, long-term chemical fertilizer application increased the abundance of nirS gene by 426%. However, partial replacement of chemical fertilizer by organic materials decreased the abundance of nirS gene. The abundance of nosZ I gene was one order of magnitude higher than that of nosZ II, indicating the domination of nosZ I in the acidic Ultisol. Long-term fertilization did not affect the abundance of nosZ II, whereas chemical fertilizer+pig manure increased the abundance of nosZ I by 138%. Results of stepwise regression analysis showed that available phosphorus content was the primary factor regulating the abundance of nosZ I gene, whereas the abundance of the nosZ II gene was mainly regulated by nitrate content. Moreover, the lowest (nirS+nirK)/(nosZ I+nosZ II) value in the chemical fertilizer+pig manure treatment indicated that long-term manure application might reduce N2O emission potential in Ultisols.