Lessons learned in the long term

 Roland J. Buresh   |  
A n irrigated rice field in Laos. (Photo: IRRI)

An irrigated rice field in Laos. (Photo: IRRI)

The most intensively cropped experimental site in Asia began four decades ago largely as a demonstration plot conveniently close to IRRI’s administrative buildings. Researchers recognized its potential as an outdoor laboratory, and today the Long-Term Continuous Cropping Experiment (LTCCE) is a treasure for researching the sustainable management of intensive irrigated rice ecosystems.

Between the first planting on 18 February 1963 and the middle of 2002, the LTCCE produced 115 rice crops on its single hectare — two crops annually until 1968, then three crops per year using short-duration modern varieties. The LTCCE is thus a prototype of the irrigated rice ecosystems that have spread across Asia and become increasingly vital to food security.

Irrigated fields producing two or three crops of rice per year now account for more than 40% of world rice production. One and a half billion rice farmers and consumers depend on their sustainable productivity. The LTCCE provides early insights into the long-term effects of  intensive cropping on these ecosystems and how best to maintain their resource base and productivity.

We manage the LTCCE to achieve high and stable yields on a sustainable basis, with annual grain production for the three rice crops reaching 17 t/ha. We examine high-yielding varieties and elite breeding lines in combination with optimal nitrogen (N) fertilizer and crop management. We research the soil’s microbial dynamics, organic matter, carbon sequestration, and chemical and biological changes, as well as cultivar micronutrient content, aquatic arthropods, and interactions between N fertilizer and rice diseases.

Through the LTCCE, we have learned that high rice yields remain attainable after intensive cropping for more than 30 years. A rice yield of 8.9 t/ha in the 2002 dry season with optimal N fertilizer management was the fifth highest to date in the history of the experiment. It was made possible by favorable climatic conditions. Climate, particularly solar radiation, is the dominant factor affecting long-term yield trends when N and crop management are optimized for high yield.

Much of the N in high-yielding rice comes from indigenous soil organic matter and biological N2 fixation, but supplemental N from fertilizer is essential. The optimal amount is directly related to plant need, which rises in high-yielding seasons. With the “best” N fertilizer practice historically used in the LTCCE (and typically recommended to Asian rice farmers), the rate of N fertilizer for a given season remains constant from year to year. We have learned, however, that rice yields can be further improved with need-based management. This means applying N to “feed” rice only when it is “hungry” because the N content in plant tissue has fallen to a critical level.

Farmers can quickly and easily gauge this level with simple tools such as the chlorophyll meter or the inexpensive leaf color chart. In the 2001 and 2002 dry seasons, need-based N management guided by the leaf color chart resulted in higher rice yields than did the “best” N practice. We achieved this yield improvement by better adjusting N fertilizer use to the growing season and climatic conditions — without decreasing the fertilizer’s effectiveness (yield per amount added). This demonstrates the potential for knowledge-based N management practices to further boost yield, largely through adjusting crop management to seasonal and site-specific conditions.

We have learned that intensive rice ecosystems can maintain soil organic matter, levels of which in the LTCCE have actually increased slightly (5–10%) in the past 15 years.

This is remarkable because we remove all above ground crop residues. Aquatic organisms and biological processes unique to submerged soils ensure sustained soil organic matter and sequestration of soil carbon.

We have learned that the capacity of irrigated rice ecosystems to supply indigenous N to rice can be sustained under intensive cropping. The yield in LTCCE plots not receiving N fertilizer has remained constant for the past 25 years. In this ecosystem with relatively high inherent soil fertility, biological N2 fixation and N released from soil organic matter continue to supply a steady 50–60% of the N needed by high-yielding rice.

In sum, well-managed irrigated rice ecosystems are masterpieces of ecological vitality and sustained productivity.

Soil submergence weaves an array of biological, chemical and physical processes that make these ecosystems unique in agriculture. Our understanding of their ecological marvels, which the LTCCE continues to expand, helps us to refine natural resource management to further enhance productivity and sustainability in an environment-friendly fashion.

Roland J. Buresh is a soil scientist.

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