Long-Term Soil Ecosystem Studies

Much of the world’s food supply is produced in intensively managed cropping systems that involve high inputs and high outputs. As our populations continue to increase while the amount of arable lands steadily shrinks, these intensive systems assume greater importance. Can they be maintained into the indefinite future? Will they continue to supply high yields of our foodstuffs? Answers can be gained solely through long-term field studies.

As an example, two or even three annual crops of irrigated lowland (paddy) rice are now the norm in favorable lands of tropical Asia, and such intensive cropping produces much of the regional rice supply. When this system became possible in the 1960s with the development of rapidly maturing, semi-dwarf rice varieties (through the “Green Revolution”), long-term field experiments were soon begun to evaluate the sustainability of grain yields and soil health under multiple annual cropping. The results for one triple-cropped field study in the Philippines are depicted in the figure as grain yield response to nitrogen (N) fertilizer input. In the early years of the field study (1970-1972, upper curve), grain yield was about 6 tons ha-1 without fertilizer N input, and with increasing fertilizer application grain yield approached the yield potential of 10 t ha-1. After 19 years of continuous triple-cropping, this yield response had declined substantially in a near parallel manner (bottom curve), creating a yield loss of about 3 t ha-1 at the highest fertilizer rate. Graphically the yield loss can be attributed to the lowered starting point of the 1989-91 curve, namely the grain yield that was supported by native soil N without fertilizer N input. The nearly equal slopes of the two curves indicate that the 1989-91 crop responded similarly to fertilizer N input as in 1970-72, so the yield decline was not caused by a change in intrinsic plant characteristics.

These long-term trends strongly suggest the cause of the yield decline as decreased supply of soil N. Yet the quantity of soil N had not changed during the 19 years of intensive cropping, so this graph became the basis for directing subsequent research at changes in the quality, or chemical nature, of soil N.

Additional evidence for the central role of N in the yield decline is shown in the dotted curve (1992-93). When the role of N in the yield decline was first hypothesized in 1991, N fertilizer was applied more frequently and at greater total rates to better synchronize fertilizer N with plant N demand. The resulting boost in fertilizer N availability compensated for the decreased availability of soil N, enabling reestablishment of the original yields from 1970-72—a reversal of the yield decline.

Careful long-term management of this field study and the recording of key agronomic and soil data enabled a process-level evaluation of system sustainability, leading to mitigation options that will maintain the high productivity of this vital cropping system into future years.

Dan Olk, USDA-ARS

Reference
Cassman K.G., S.K. De Datta, D.C. Olk, J. Alcantara, M.I. Samson, J. Descalcota, and M. Dizon. 1995. Yield decline and the nitrogen economy of long-term experiments on continuous, irrigated rice systems in the tropics. p. 181-222 In R. Lal and B.A. Stewart (eds) Soil management: Experimental basis for sustainability and environmental quality. CRC/Lewis Press, Boca Raton, FL.