Carbon Storage in Soils Under Continuous Cereal Cropping: N Fertilizer and Straw



E.D. Solberg1, M. Nyborg2, R.C. Izaurralde2, H.H. Janzen3, S.S. Malhi4, and M. Molina-Ayala2
1Agronomy Unit, Alberta Agriculture, Food and Rural Development, Edmonton, AB, T6H 4P2;
2Department of Renewable Resources, University of Alberta, Edmonton, AB, T6G 2E3;
3Agriculture and Agri-Food Canada, Lethbridge Research Centre, P.O. Box 3000, Lethbridge, AB, T1J 4B1;
4Agriculture and Agri-Food Canada, Lacombe Research Centre, 6000 C & E Trail, Lacombe, AB, T4L 1W1

Problem

A field experiment with continuous barley grown on a Gray Luvisolic soil was started 13 years ago to determine if yield with straw retention, versus disposal, would increase over the years. The experiment had different rates of fertilizer N and, consequently, the influence of both straw and N on yield was measured as the experiment progressed. As well, the experiment gave an excellent opportunity for monitoring annual changes in soil carbon (C) and nitrogen (N) over 13 years.

Literature Review

Our results for central Alberta indicated that after 11 years, the C content in the 0 to 6 inches depth in a Gray Luvisolic was greater by 3560 to 8900 lbs of C per acre (increasing soil C content by 14 to 31%), where 45 lbs of fertilizer N per acre was applied yearly.(5) In a similar field experiment on a Black Chernozemic soil, the addition of N had little effect on soil C, but the retention of straw, versus the removal of straw, resulted in 4450 to 6230 lbs C per acre (increasing soil C content by 6 to 8%) when 45 lbs N per acre was used each year. The increasingly large amounts of stored C that we recorded after only 11 years, are unusual in the literature. After 29 years on a thin Black Chernozem at Indian Head,(2) and after 31 years on an Orthic Black Chernozem at Melfort,(3) the application of N on continuous wheat did not significantly increase the amount of C in the soil. However, Janzen(4) reported a 14% increase in soil C after 18 years of N fertilization on a Dark Brown Chernozem soil at Lethbridge.

Cropping frequency, and hence straw addition, generally affect soil C content. In a review article, Paustian and co-workers(6) compared data from the western United States and Sweden on the effect of rate of annually applied C on net soil C content; the results were consistent: they found that 1780 to 3560 lbs per acre of C per acre per year were required to maintain soil C input/output equilibrium. It is interesting that neither the removal nor retention of straw had an effect on the soil C content of our work on a Gray Luvisolic soil.

Study Description

The effects of Nitrogen (N) and straw disposal on both organic carbon (TOC) and light fraction carbon (LFC) soil content were examined in a field experiment. The experiment was located in central Alberta on a well-drained Gray Luvisol with a loam texture and an annual precipitation of 21.5 inches. The soil had low organic matter content (about 21 g kg-1) making addition of N fertilizer a requirement for adequate cereal crop production. The experiment was initiated in the autumn of 1982 and cropped to barley (Hordeum vulgare L.) from 1983 to 1995. The experiment was tilled with either a small sweep-chisel cultivator or a rototiller, once or twice for preparing the seedbed in spring, and once or twice after harvest in autumn. The experiment had 14 treatments replicated four times and each of the 56 sub-plots was 9' by 22' in area. There were four rates of N (0, 22, 45 and 67 lbs per acre applied as urea in each spring) and two methods of handling straw (discarding versus retaining). Only 8 treatments are considered in this report. In the autumn of 1995, 8 soil cores (1.3" in diameter) were taken from each sub-plot, cut into 4 segments (0-3, 3-6, 6-9 and 9-12 inches) and composited by depth intervals. The analyses of soil samples included: bulk density, total organic carbon (TOC), total N, light fraction carbon (LFC), and light fraction nitrogen. Results were corrected for differences in soil bulk density among the various treatments.

Major Findings

The soil was unusually deficient in N. The average yield of barley from 1983 to 1994 showed pronounced response to N. Without N fertilizer, the average yield was only 19 bus/ac re, compared with 51 bus/acre with N at 67 lbs/acre/year (Table 1). Similarly, for treatments with straw retention, yield was 21 bus/acre without N but rose to 57 bus/acre with N at 67 lbs/ac/year. Soil bulk density decreased with the increase in applied N. The TOC content in the first two soil layers was 18,601, 21,271, 23,585, and 24,920 lbs/acre for treatments receiving N at 0, 22, 45, and 67 lbs/acre/year, respectively (Table 2). Total N concentration behaved in the same manner as did organic C in the top 2 soil layers, although the values for total N were only about one-tenth as great. The apparent storage of C in soil resulting from fertilizer N applied annually at 22, 45 and 67 lbs/acre/year during 12 years (1983-1995) was 2581, 4984 and 6319 lbs/acre as TOC, respectively. Fertilizer N was more effective than straw in storing C in the soil. The apparent storage of C for treatments where straw had been discarded, fell to within 2/3 and 3/4 of that determined for treatments where straw had been retained (Table 2).

Table 1. Average annual barley yield (1983 to 1994) of grain and straw, and bulk densities (1995)
No. Treatment Annual yield Bulk density
fertilizer N
(lbs/acre)
Straw Grain
(bus/acre)
Straw
(lbs/acre)
0-7.5 cm
(kg L-1)
7.5-15 cm
(kg L-1)
1 0 Off 19.1 943 1.32 1.57
2 0 On 19.2 979 1.24 1.49
3 22 Off 34.2 1458 1.24 1.49
4 22 On 32.9 1638 1.22 1.45
5 45 Off 44.5 2296 1.20 1.44
6 45 On 43.5 2172 1.17 1.38
7 67 Off 50.9 2723 1.17 1.40
8 67 On 51.3 2617 1.17 1.44

Table 2. Total organic C and total N in two combined top layers of soil after 12 years, and apparent increase in C storage related to fertilizer N and straw handling
No Treatment Increase in C storage from N and straw use*
fertilizer N
(lbs/acre)
Straw Total organic C
(lbs/acre)
Total N
(lbs/acre)
Total organic C
(lbs/acre)
Light fraction of organic C
(lbs/acre)
1 0 Off 18690 2002 a b
2 0 On 18601 2038 a b
3 22 Off 20559 2181 1869 223
4 22 On 21271 2198 2581 303
5 45 Off 21894 2207 3204 418
6 45 On 23585 2367 4984 454
7 67 Off 23496 2385 4806 490
8 67 On 25009 2483 6319 632

*The increase in C storage was calculated by deducting the treatments without N (No. 1 or No. 2) from treatments with N (No. 3, 4, 5, 6, 7, and 8).
a Amounts of total organic C in treatments No. 1 and 2 were 18690 and 18601 lbs/acre, respectively.
b Amounts of light fraction of organic C in treatments No. 1 and 2 were 623 and 676 lbs/acre, respectively.

The results point to the influence of cereal roots in increasing soil C. The apparent amounts of stored TOC were surprisingly large considering the moderate amounts of N applied (Table 3). The calculated ratios of C stored (lbs/acre) to applied N (lbs/acre) were 9, 9, and 7 for N rates of 22, 45, and 67 lbs/acre, respectively. The LFC was approximately one-tenth of TOC in the stored C, but the amount of LFC in the stored C was nearly as much as that found in the bulk of the non-sequestered C (Table 2). The LFC was 3 to 4% of the TOC in the non-N treatments, compared to 9 to 24% of soil organic C found by Bremer and co-workers on a Dark Brown soil. In addition, they found N to have no effect on LFC. These results suggest the merit of considering LFC as a sensitive measure for detecting gains in soil C. The main conclusion from this 12-year experiment was that N fertilizer had a marked effect on increasing C storage in a soil low in organic matter.

Table 3 The total accumulation of applied fertilizer N and increase in stored C after 12 years.
No. Treatment Total application of fertilizer N (1983-1995) Increase in carbon straw from fert. N use (1983-1994) Apparent increase in stored C from 1983 to 1995 Ratio of stored C to fert. N applied
fertilizer N Straw
lbs/acre . lbs/acre lbs/acre lbs/acre .
3 22 Off 289 0 1869 6.5
4 22 On 289 3480 2581 8.9
5 45 Off 579 0 3204 5.5
6 45 On 579 6292 4984 8.6
7 67 Off 868 0 4984 5.5
8 67 On 868 8642 6319 7.3
*The value was 4,610 lbs C/ acre for treatment No. 2 which did not receive N. The values shown for treatments No. 4, 6 and 8 are not values with deductions of 4,610. As well, carbon content of straw was estimated as 44%.

Applied Questions

Farmers are told that annual cropping reduces soil organic matter. Do these results agree?
Not at all. At least in this low organic matter soil, the annual addition of moderate rates of nitrogen fertilizers contributed to a significant storage of C from the atmosphere.

Most of the increase in C storage from fertilizer N occurred even when straw was removed. What is the likely reason for this observation?
We propose that an important source of carbon to either maintain or increase soil carbon is derived from roots which, in turn, is enhanced by the addition of nitrogen fertilizer.




References

1. Bremer, E., Janzen, H.H., and Johnston, A.M. 1994. Sensitivity of total, light fraction and mineralizable organic matter to management practices in a Lethbridge soil. Can. J. Soil Sci 74:131-138.
2. Campbell, C.A., Biederbeck, V.O., Zentner, R.P., and Lafond, G.P. 1991a. Effect of crop rotations and cultural practices on soil organic matter, microbial biomass and respiration in a thin Black Chernozem. Can. J. Soil Sci. 71:363-376.
3. Campbell, C.A., Bowren, K.E., Schnitzer, M., Zentner, R.P., and Townley-Smith, L. 1991b. Effect of crop rotations and fertilization on soil organic matter and some biochemical properties of a thick Black Chernozem. Can. J. Soil Sci. 71:377-387.
4. Janzen, H.H. 1987. Effect of fertilizer on soil productivity in long-term spring wheat rotations. Can. J. Soil Sci 67:165-174.
5. Nyborg, N., Solberg, E.D., Malhi, S.S. and Izaurralde, R.C. 1995 Fertilizer N, crop residue, and tillage alter soil C and N content in a decade. In R. Lal et al (eds.) Soil Management and Greenhouse Effect. Lewis Publishers. p. 93-100.
6. Paustian, K. Robertson, G.P. Elliott, E.T. 1995. Management impacts on carbon storage and gas fluxes (CO2, CH4) in mid-latitude cropland. In R. Lal et al (eds.) Soil Management and Greenhouse Effect. Lewis Publishers. p. 69-83.
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