Source of nitrous oxide emissions during the cow manure composting process as revealed by isotopomer analysis of and amoA abundance in betaproteobacterial ammonia-oxidizing bacteria

Abstract

A molecular analysis of betaproteobacterial ammonia oxidizers and a N(2)O isotopomer analysis were conducted to study the sources of N(2)O emissions during the cow manure composting process. Much NO(2)(-)-N and NO(3)(-)-N and the Nitrosomonas europaea-like amoA gene were detected at the surface, especially at the top of the composting pile, suggesting that these ammonia-oxidizing bacteria (AOB) significantly contribute to the nitrification which occurs at the surface layer of compost piles. However, the (15)N site preference within the asymmetric N(2)O molecule (SP = delta(15)N(alpha) - delta(15)N(beta), where (15)N(alpha) and (15)N(beta) represent the (15)N/(14)N ratios at the center and end sites of the nitrogen atoms, respectively) indicated that the source of N(2)O emissions just after the compost was turned originated mainly from the denitrification process. Based on these results, the reduction of accumulated NO(2)(-)-N or NO(3)(-)-N after turning was identified as the main source of N(2)O emissions. The site preference and bulk delta(15)N results also indicate that the rate of N(2)O reduction was relatively low, and an increased value for the site preference indicates that the nitrification which occurred mainly in the surface layer of the pile partially contributed to N(2)O emissions between the turnings.

FIG. 1.

Sampling points of the piles. Samples were taken just before each turning.

FIG. 2.

(a) N₂O emission profile of pile 1. The arrows indicate the turnings. The gray dotted line indicates the detection limit. (b) δ¹⁵N^bulk, δ¹⁸O, and site preference (SP) of N₂O. Open triangles indicate δ¹⁵N^bulk, open squares indicate δ¹⁸O, and closed circles indicate the site preference. The standard site preferences of N₂O from nitrification (33‰) and denitrification (0‰) are also indicated (44).

FIG. 3.

NH₄⁺, NO₂⁻, and NO₃⁻-N profiles of the compost piles. Open bars indicate nitrite, dotted bars indicate nitrate, and closed bars indicate ammonium. The error bars indicate the standard deviations (n = 3). A to E indicate the sampled zones of the pile, described in Fig. 1.

FIG. 4.

16S rRNA (open bars) and amoA (closed bars) copy numbers g⁻¹ (dry weight [DW]) compost for each zone of the compost piles. Error bars indicate the standard deviation for three replicate DNA extractions.

FIG. 5.

Temperature profiles at the top and core of pile 2 and the ambient air. The arrows indicate the turnings.