Effects of free-air CO 2 enrichment and nitrogen supply on the yield of temperate paddy rice crops

Abstract

Increasing atmospheric CO 2 concentrations [CO 2] are generally expected to enhance photosynthesis and growth of agricultural C 3 cereal crops and as a result substantially increase yields. However, little is known about the combined effect of elevated [CO 2] and nitrogen (N) supply on grain yield. To better understand the interactive effects of these factors on the yield of rice ( Oryza sativa L.), we conducted a free-air CO 2 enrichment (FACE) experiment at Shizukuishi, Iwate, Japan, in 1998–2000, using the japonica cultivar Akitakomachi. The target [CO 2] of the elevated [CO 2] plots was 200 ppmV above that in the ambient air. Three levels of N were supplied: low (LN, 4 g N m −2), medium (MN, 8 (1998) and 9 g N m −2 (1999, 2000)) and high N (HN, 12 (1998) and 15 g N m −2 (1999, 2000)). The MN level was similar to that recommended to local farmers. Across the 3 years, there was a positive [CO 2]×N interaction for grain yield, with yields increasing only 7% with LN but about 15% with MN and HN. A similar interaction was found for fertile spikelet number per square meter. Across N levels and years, the [CO 2]-induced increases in yield were closely related but less than the increases in total dry matter production: as a result harvest index (HI) decreased slightly with elevated [CO 2]. This decrease in HI was due in part to a reduction in the number of productive (panicle bearing) tillers relative to total tiller number with elevated [CO 2]. Yield increases were related most strongly to greater spikelet number per unit ground area, which in turn were due to increases in both panicle number per square meter and spikelet number per panicle. Small increases in individual grain mass with elevated [CO 2] had only a minor effect on the overall yield increases. The spikelet number response to elevated [CO 2] was limited with LN, but we found no evidence that N supply at levels greater than that recommended (i.e. above the MN level of N supply) resulted in further increases in spikelet number with elevated [CO 2]. This suggests that for the cultivars and conditions of this experiment, the [CO 2]-induced increases in yield will approach a ceiling at the recommended rates of N supply.