de Jong, E.

Kirkland, J.A.

English

University of Saskatchewan Research Archive

EFFECTS OF SOIL MANAGEMENT ON DENITRIFICATION J. A. Kirkland and E. de Jong Saskatchewan Institute of Pedology, University of Saskatchewan Saskatoon, Sask. IN1RODUCTION Denitrification, the reduction ofNOJ--N to the gases N20 and N2 and their subsequent loss to the atmosphere, is an anaerobic process. However significant denitrification losses have been observed on seemingly well-aerated Saskatchewan soils (Aulakh 1983, Colaco 1979). These losses have been attributed to the existence of anaerobic microsites (Norstadt and Payne 1984). Anaerobic microsites form when the physical and microbial conditions are such that the supply of oxygen to a microsite in the soil cannot meet the demand. This may arise from intense microbial activity at a point within the soil or slow gaseous diffusion rates within large water-saturated aggregates (Galsworthy et al. 1978, Smith 1977). The occurrence of anaerobic microsites and, hence, denitrification depends on physical and microbial soil properties such as water content, air porosity, aggregate size, temperature, soil respiration and substrate levels. Denitrification has been shown to vary between fallow and cropped land and between conventionally and zero-tilled soils (Aulakh et a1. 1982,1984). The objective of this study is to relate variations between management systems to soil properties and to obtain estimates of annual denitrification in the Black soil zone. ME1HODS Denitrification and soil properties were monitored on long term management plots at the Agriculture Canada Research Station at Melfort, Saskatchewan. The soil is an orthic Black chernozem (Melfort silty clay). The treatments studied were conventionally and zero-tilled plots in a wheat fallow rotation. A more detailed description of the treatments is given in Selles et al. ( 1984 ). The site was sampled five times between May and October in 1986 and six times between April and September in 1987. Measurements were made at three different depths; 2.5-10 em, 10-17.5 em and 17.5-25 em, and were replicated three times. Denitrification rates were measured using the acetylene inhibition method of Aulakh (1983). Moisture content, air porosity and soil respiration measurements were made on core samples. Soil respiration was also calculated from field measurements of C02 concentrations in gas sampling tubes using the method of de Jong and Schappert (1972). Soil temperature was measured using thermocouples installed at the sampling depths. Bulk soil samples were taken and standard analyses for N03-, N114+ and water-soluble carbon were performed. Daily precipitation and air temperature data were obtained from the Environment Canada Meteorological Station on the research farm. The effects of management on denitrification were examined using analysis of variance and protected least significant differences. Regression analysis was used to study the relationships between denitrification and the soil properties. A logarithmic transformation was applied to denitrification and soil respiration values to normalize their distribution. All other variables exhibited normal distributions. RESULTS AND DISCUSSION Estimated annual denitrification losses are shown in Table 1. Losses range from 104 kg/ha from the chemical summerfallow in 1987 to 9 kglha from the zero till wheat in 1987. The rate of denitrification was highest in both years on the chemical (zero-till) 184 sumrnerfallow plot and it was significandy (P>O.OS) greater than denitrification on the other plots. Fallow treatments had significantly higher denitrification than cropped treatments in 1987 (P>O.OOl), but there were no significant differences between cropped zero and conventionally tilled plots. Table 1. Estimated annual denitrification losses. Treatment ZF (Zero-till Fallow) CF (Conventional till Fallow) zw (Zero-till Wheat) CW (Conventional till Wheat) 1986 72 30 37 37 Denitrification kgN/ha/yr 1987 104 61 9 16 The pattern of denitrification throughout the sampling period is shown in Figure 1 for 1986 and Figure 2 for 1987. In 1986 the greatest denitrification losses occurred between May 29th and July 22nd. At this time all plots lost similar amounts of nitrogen through denitrification (30 kglha), but only the chemical fallow continued to experience significant denitrification until August 28th. This was attributed to the high surface moisture on the chemical fallow plot (42% v/v) relative to the tilled fallow (37% v/v) and the cropped plots (37 and 33% v/v for zero and conventional till respectively). Denitrification in 1987 did not peak until August and was largely confmed to the fallow plots. This was a consequence of the dry spring. In 1987 there were no rainfall events greater than 10 mrn between April 15th and July 1Qtb and surface moisture contents were below 35% v/v. The lack of denitrification on the cropped plots in August was a result of plant water use; the surface moisture contents in these plots averaged 37% v/v. 120 • MAY 29 -JULY 22 '2 ..c: r?lJ JULY 22 - AUG. 28 ~100 II AUG. 28 - SEPT. 29 ~ 80 D SEPT. 29 - OCT. 20 ~ 60 i 40 20 ~ 0 Figure 1. Temporal variation in denitrification in 1986 (see Table 1. for treatment names). 185 ZF CF 'Z:N TREATMENT • APRIL15-MAY5 ~ MAY5-JUNE11 rl JUNE 11-JULY 27 0 JULY 27- AUG. 27 II AUG. 27 - SEPT. 24 Figure 2. Temporal variation in denitrification in 1987 (see Table 1. for treatment names). Volumetric water content (VWC) correlated well with denitrification (N-LOSS) and explained 26% of its variability. Gravimetric water content (GRA V), air porosity (AIRP),percentage of large aggregates (AGS), temperature (TEMP) and soil respiration (RESP) were also significantly correlated to denitrification. No relationship was found between denitrification and the substrate variables, soluble carbon (SOLC) and nitrate (N03) which suggested that substrate levels were not limiting. Two linear regression models were used to relate denitrification to soil properties. The model: ln N-LOSS = 0.147 VWC + 0.088 TEMP+ 0.091 AGS - 5.792 explained 46% of the variability in denitrification. In this model the VWC is an indicator of soil aeration status and moisture supply, TEMP is related to microbial activity and AGS is a measure of the suitability of the structure for anaerobic microsite formation. An alternative model: InN-LOSS= 9.371 GRA V- 0.057 AIRP + 0.631ln RESP +0.111 AGS- 3.919 also includes the AGS term but has replaced VWC with AIRP and GRA V which represent aeration and moisture supply as separate parameters. RESP is used in place of temperature as an indicator of microbial activity. This model explains 44% of the variability. The model fit can be improved by splitting the observations into depth categories (Table 2). The model for 2.5-10 em depth is similar to the first model but the aggregate term is not significant. The best relationship is found at the 17.5-25 em depth where substrate availability appears to influence denitrification. 89% of the variability is explained. The worst relationship occurs at the 10-17.5 em depth where aggregate size is the only significant variable and only 28% of the variability is explained. It appears that there is some depth related factor that is not adequately characterized by the soil properties considered in this study. Some further improvement in model fit might be obtained by constructing a more mechanistic model. 186 Table 2. Model variables for the different depths. Depth 2.5-10 em 10-17.5 em 17.5-25 em Variables Significant in Model % Variance Explained vwc,~ 49 AGS 28 GRAY, RESP, AGS, SOLC, N03 89 CONCLUSIONS Denitrification losses were estimated to range from as high as 104 kglha/yr from chemical fallow to 9 kg/ha/yr from zero till wheat Fallow soils lost significantly more nitrogen through denitrification than cropped soils but there were no significant differences between conventionally and zero tilled plots in the crop year. Volumetric water content explained 26% of the variability in denitrification and the highest denitrification rates were observed following rains of greater than 10 mm. Air porosity, aggregate size, temperature and soil respiration were also related to denitrification and explained 45% of the variability when used along with moisture content in linear models. Substrate availability was adequate in the top 17.5 em but was limiting denitrification in the 17.5-25 em layer. REFERENCES AULAKH, MS. 1983. Gaseous N losses from soils under various cropping and tillage systems. Ph.D thesis. University of Saskatchewan, Saskatoon, Sask. AULAKH, M.S., D.A. RENNIE and E.A. PAUL. 1982. Gaseous nitrogen losses from cropped and summerfallowed soils. Can. J. Soil Sci. 62:187-195. AULAKH, M.S., D.A. RENNIE and E.A. PAUL. 1984. The influence of plant residues on denitrification rates in conventional and zero tilled soil. Soil Sci. Soc. Amer. J. 48:790-794. COLACO, V. 1979. Denitrification in soils under a wheat-fallow rotation. Ph.D thesis. University of Saskatchewan, Saskatoon, Sask. DE JONG, E. and H. G. V. SCHAPPERT. 1972. Calculation of soil respiration and activity from C02 profiles in the soil. Soil Science 113:328-333 GALSWORTHY, A.M., J.R. BURFORD and D.J. GREENLAND. 1978. Modelling rates of nitrification and denitrification in soil. In. D.R. Nielsen and J.G. MacDonald. (eds). Nitrogen in the Environment Vol. 2. Academic Press, New York, USA. NORSTADT, F.A. and B.F. PAYNE. 1984. Water stable microaggregates: Anaerobic microsites in well-drained semiarid soils. Soil Sci. Soc. Amer. Conference. November 1984, Las Vegas, USA. SELLES, F., R.E. KARAMANOS and K.E. BOWREN. 1984. Changes in naturai15N abundance of soils associated with tillage practices. Can. J. Soil Sci. 64:345-354. SMITH, K.A. 1977. Soil aeration. Soil Science 123:284-291. 187 

Effects of soil management on denitrification

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Effects of soil management on denitrification

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