Role of plant residues in determining temporal patterns of the activity, size, and structure of nitrate reducer communities in soil

Abstract

The incorporation of plant residues into soil not only represents an opportunity to limit soil organic matter depletion resulting from cultivation but also provides a valuable source of nutrients such as nitrogen. However, the consequences of plant residue addition on soil microbial communities involved in biochemical cycles other than the carbon cycle are poorly understood. In this study, we investigated the responses of one N-cycling microbial community, the nitrate reducers, to wheat, rape, and alfalfa residues for 11 months after incorporation into soil in a field experiment. A 20- to 27-fold increase in potential nitrate reduction activity was observed for residue-amended plots compared to the nonamended plots during the first week. This stimulating effect of residues on the activity of the nitrate-reducing community rapidly decreased but remained significant over 11 months. During this period, our results suggest that the potential nitrate reduction activity was regulated by both carbon availability and temperature. The presence of residues also had a significant effect on the abundance of nitrate reducers estimated by quantitative PCR of the narG and napA genes, encoding the membrane-bound and periplasmic nitrate reductases, respectively. In contrast, the incorporation of the plant residues into soil had little impact on the structure of the narG and napA nitrate-reducing community determined by PCR-restriction fragment length polymorphism (RFLP) fingerprinting. Overall, our results revealed that the addition of plant residues can lead to important long-term changes in the activity and size of a microbial community involved in N cycling but with limited effects of the type of plant residue itself.

FIG. 1.

(A) Dynamics of potential nitrate reduction rates in control plots and in plots amended with wheat, rape, or alfalfa residues. For each date, the same letters above the bars (means ± standard deviations; n = 9) indicate that treatments were not significantly different according to a Student's t test (P < 0.05). Rainfall (□) and temperature (×) data are indicated for each sampling date. (B) Percentage of changes from the control in the residue-amended plots. For the same residue, identical letters above the bars (means ± standard deviations; n = 9) indicate that the sampling times were not significantly different according to a Mann-Whitney test (P < 0.05).

FIG. 2.

(A) Dynamics of the estimated size of the nitrate-reducing community using the narG gene as a molecular marker in control plots and in plots amended with wheat, rape, or alfalfa residues. For each date, the same letters above the bars (means ± standard deviations; n = 9) indicate that treatments were not significantly different according to a Student's t test (P < 0.05). (B) Percentage of changes from the control in the residue-amended plots. For the same residue, identical letters above the bars (means ± standard deviations; n = 9) indicate that the sampling times were not significantly different according to a Mann-Whitney test (P < 0.05).

FIG. 3.

(A) Dynamics of the estimated size of the nitrate-reducing community using the napA gene as a molecular marker in the control plots and in plots amended with wheat, rape, or alfalfa residues. For each date, identical letters above the bars (means ± standard deviations; n = 9) indicate that treatments were not significantly different according to a Student's t test (P < 0.05). (B) Percentage of changes from the control in the residue-amended plots. For the same residue, identical letters above the bars (means ± standard deviations; n = 9) indicate that the sampling times were not significantly different according to a Mann-Whitney test (P < 0.05).

FIG. 4.

Principal-component analysis of the structure of the nitrate reducer community using the narG gene in the control plots (C) (white) and in plots amended with wheat (W) (light gray), rape (R) (dark gray), or alfalfa (A) (black) residues. (A) Data for 4 July 2006; (B) data for 26 July 2006; (C) data for 20 December 2006; (D) data for 15 March 2007. Statistical ellipses drawn over the plot replicates represent 90% confidence. The percentages of explained variations for the first two axes are indicated within the figures.

FIG. 5.

Principal-component analysis of the structure of the nitrate reducer community targeted using the napA gene in the control plots (C) (white) and in plots amended with wheat (W) (light gray), rape (R; dark gray), or alfalfa (A) (black) residues. (A) Data for 4 July 2006; (B) data for 26 July 2006; (C) data for 20 December 2006; (D) data for 15 March 2007. Statistical ellipses drawn over the plot replicates represent 90% confidence. The percentages of explained variations for the first two axes are indicated within the figures.