**5. Sustainable development and nutrient management of red soil**

Red soils are usually low in pH, nutrients, and organic matter content, and difficult to cultivate because of its poor physical condition and low water-holding capacity. Therefore, the pro‐ ductivity of the red soils is low under natural conditions. Most farmers have livestock or poultry production on the side in the red soil region, and they use the grain to feed the animal and use the manure to improve soil productivity. Although combined application of chemical fertilizer and rice straw or pig manure was an effective method to improve soil fertility (**Figures** **3** to **5**), soil macro-aggregates (**Figures 6** and **7**), and soil organic matter content (**Figure 2**), it could result in N and P accumulation and soil acidification in the long term (**Figure 1**). The accumulation of P in the soil may result in high risk of P losses to the environment [16]. Therefore, it is imperative to monitor nutrient balance when large quantities of organic amendments are used. This study showed that it only took 4–5 years of applying phosphate fertilizer alone, 2 years when organic-chemical fertilizers were applied simultaneously to increase Olsen-P to over 20 mg P kg−1 (sufficiency level) (**Figure 4**). When soil P is sufficient for plant growth, continuous application of P fertilizer including organic manure not only hurt farmer's economic return but also contributing to non-point source pollution. Therefore, better nutrient management strategy needs to be developed and disseminated to farmers in the region in order to prevent potential environmental risk from crop production.

Better soil quality is generally associated with greater concentrations of soil organic matter and a plentiful supply of essential mineral elements. Thus, the recycling of organic matter and mineral elements from crop residues and animal manure to soil often benefits agricultural sustainability. Red soil production under long-term fertilization is perceived to be environ‐ mentally friendly, but P accumulation and acidification in the PM may lead to faster degra‐ dations of soil quality than in the CK and may be an important cause of eutrophication in water bodies (**Figures 1** and **4**).

Soil acidification is an important cause of soil degradation. Generally, soil acidification is a slow process under natural conditions over hundreds to millions of years, because soils are strongly buffered by ion exchange reactions, the weathering of soil minerals, and interactions with aluminum and iron in the acidic range [17]. However, this process in red soils is acceler‐ ated by the addition of crop straw and animal manure; and soil acidification appeared only after 7 cultivation years (**Figure 1**). Generally, strategies for the application of organic manure have been based on meeting crop N needs to maximize plant growth and minimize nitrate loss by leaching, a potential groundwater contaminant [18]. Guo et al. (2010, [19]) showed that severe soil acidification in China's croplands occurred, and attributed it to the combination of high-N fertilizer inputs, plant uptake, and removal of base cations from soils, and acid deposition, with the dominant effect from the overuse of N fertilizer. But, soil is a mixture of acid/base systems, so there are many causes of soil acidification in addition to N-induced acidification.

Most of organic manures often contain significant amounts of low-molecular-weight organic acids due to decomposition of large quantities of organic matter input, which could be another reason for the accelerated P accumulation and soil acidification [16]. Many organic acids contain carboxyl and hydroxyl groups, and possess negative charge, which strongly competes for the adsorption sites with phosphate [20, 21]. Manure can also change soil pH and thus alter soil P availability. Guo et al. (2010, [19]) pointed out that soil could be acidified by the excessive application of farmyard manure. But the mechanisms of manure-induced P accumulation and P accumulation-induced soil acidification still need further investigation [18], especially in the red soil with strong P fixation capacity. Maintaining stable levels of production and quality under fertilization without compromising economic profitability or the environment is an important guarantee of agricultural sustainability. Therefore, new strategies of using both commercial fertilizers and animal manure must be established to prevent P buildup and to minimize acidification in the red soil region.
