化肥对中国粮食产量变化贡献率的研究

【目的】化肥在中国粮食产量的增加中发挥了重要作用，但中国单位面积化肥施用量已远超其他国家。虽然田间试验已证明减少化肥施用量不会导致粮食产量大幅下降，但部分经济学者和粮食生产者对减少化肥施用量仍持谨慎态度。科学评价我国现阶段化肥对粮食产量变化的贡献率，对国家层面制定合理的化肥施用决策有重要的参考价值。已有研究侧重于利用时间序列模型分析粮食产量与化肥投入量及其他因素的关系，而不同省份粮食生产力存在差异，并且化肥对粮食产量变化的贡献率是动态变化的。故本研究利用面板数据模型更好地描述中国粮食生产的投入产出关系，并更准确地反映不同阶段化肥对粮食产量变化的贡献率。【方法】研究收集了1995—2015年中国30个省份粮食投入产出数据,分别估计了柯布道格拉斯和二项式函数形式的粮食生产函数的三种面板回归模型，并利用固定效应和随机效应检验方法对不同面板模型的优劣进行评判，最终判定随机效应模型优于混合效应和固定效应模型。根据随机效应模型回归结果求解了单位面积粮食产量对化肥的施用量弹性系数，借鉴全要素生产率概念，利用粮食生产函数和化肥施用量弹性计算了化肥对单位面积粮食产量变化的贡献率，据此评价化肥施用量对粮食产量的实际影响。【结果】实证结果表明，化肥施用量的弹性系数显著，固定弹性系数为0.17，可变弹性系数呈明显的倒U形变化趋势，说明化肥投入已经进入边际报酬递减阶段，继续增加化肥施用量无法实现粮食产量的增加；但由于化肥施用量弹性系数值较高，并且大于其他要素，表明化肥的增产效力不可替代。而化肥对粮食产量变化的贡献率计算结果则表明化肥对单位面积粮食产量变化的贡献逐渐变小，趋向于0，说明化肥带来的粮食增长效应已不明显。2015年粮食投入产出数据分析结果也说明适当减少化肥施用量并不会导致粮食产量大幅减少，防灾技术、农业机械化程度的提高等其他要素对粮食产量的增加发挥了更大的作用。【结论】建议在国家层面严格控制化肥施用量，通过调整施肥方式，优化施肥结构等措施提高化肥利用效率，进一步发挥技术要素对粮食增产的作用。

Research on the contribution rate of fertilizer to grain yield in China

【Objectives】Chemical fertilizers play very important roles in the grain production of China, however, fertilizer quantity applied per unit area in China is far higher than other countries. Some field trials have shown that reducing fertilizer rate did not lead to a significant drop in grain production, some economists and grain producers remain cautious about reducing fertilizer rate. So it is important to scientifically evaluate the contribution rates of chemical fertilizers to the grain yields in China, and to make rational fertilizer application decision at the country level. The existing studies mainly analyzed the relationship between grain yield and fertilizer input as well as other factors using time series models, but the grain productivities varied from one province to the others, and the effect of fertilizers on grain yield changes is also dynamically changing. So this study chose panel data model to better depict input-output relationship of grain production in China, and reflect more accurately the contribution rates of fertilizers to grain yield changes in different stages.【Methods】Grain input-output data of 30 provinces from 1995 to 2015 were collected respectively. Three kinds of panel regression models of grain production functions in the form of C-D and binomial functions were estimated. By using methods of fixed effect test and random effect test to evaluate different panel models, the random effects model was finally judged to be better than the mixed effects and fixed effects models. Based on the regression results of random effects model, the elastic coefficients of grain yield to fertilizer application quantity per unit area were calculated. Using the concept of total factor productivity, the contribution rates of fertilizers were calculated using grain production function and fertilizer elasticity coefficients to evaluate actual impact of fertilizers on grain production.【Results】The empirical results showed that the elasticity coefficients were all significant, the fixed elastic coefficient was 0.17, and the variable elastic coefficients showed an obvious inverted U-shaped trend. It implied that the marginal fertilizer input had entered the stage of diminishing returns, continuing increase of fertilizer application quantity seemed unable to realize the increase of grain production. The elastic coefficient value of fertilizer application quantity was still high and bigger than other factors’, which demonstrated the unsubstitutability of fertilizer to grain production increase, on the other side. The contribution rates of fertilizers to grain yield increase showed that the contribution of fertilizers was diminishing and trended to zero. The input-output data analysis result of 2015 also indicated that appropriate reduction of fertilizer input would not lead to a sharp decrease in grain production, as the improvement of disaster prevention technology, agricultural mechanization degree and other elements had played greater roles in the increase of grain production.【Conclusions】It is suggested that the rates of fertilizers should be strictly controlled at the county level, and the effect of fertilizer rate on yield increase should be improved by adjusting the way of fertilization and balancing the fertilization structure, meanwhile, contribution rates of technological factors to grain yields should be further promoted.