| Soil acidification is a slow natural process accompanying soil development. However, it can be accelerated by human activities (e.g. acid rain and fertilization), which may, amongst others lead to adverse impacts on forest ecosystem health and crop production. In view of those threats, this thesis quantified acidification rates and its causes in China's forest soils and agricultural soils on the basis of element balances,and further predicted acidification trends for typical combinations cropping systems and soils. In addition,the impacts of future soil acidification on yield losses and food security were quantified in response to several designed scenarios, including changes in fertilizer and manure application and crop residue removal. The main results of the various studies are given below:1. Significant acidification has occurred in China's forest soils since 1980. Results of collected soil pH values in the early 1980s (1980-1985) and the late 2000s (2005-2010) showed an average soil pH decline of 0.36 units across China, with the most significant decrease occurring in Southwest China (0.63 units) and semi-Luvisols (0.44 units), respectively. During the past 25 years, the total acid production potential of sulfur deposition (62.5 keq ha-1) was slight greater than that of nitrogen (N) deposition (55.8 keq ha-1), while the neutralization capacity of base cation deposition was 37.1 keq ha-1. Ultimately, the deposition induced proton production was 39.3 keq ha-1, equivalent to 1.58 keq ha-1 yr-1, while the proton production induced by forest growth was 7.5 keq ha-1, equivalent to 0.3 keq ha-1yr-1. Therefore, the total acidification rates in China's forest soil were 1.87 keq ha-1 yr-1, of which 84% could be contributed to atmospheric deposition and 16% to forest growth. Our results thus showed that atmospheric deposition was the main driver of forest soil acidification.2. On the basis of major elements input-output budgets, soil acidification rates (actual acidification and potential acidification) and the causes were evaluated in this study. Results showed that N induced proton production increased from 4.7 keq ha-1 yr-1 in 1980 to a peak of 11.0 keq ha-1 yr-1 in 1996 and a nearly constant value of approximately 8.6 keq ha-1 yr-1 after 2000. The proton production induced by base cation removal in crops increased from 1.2 to 2.3 keq ha-1 yr-1, thus causing a total proton production increase from 5.9 to 10.9 keq ha-1 yr-1 in the period 1980-2010. As a result, the actual acidification rates reflect by base cation(BC) losses accelerated from 3.1 to 6.6 keq ha-1 yr-1 and potential acidification rates reflected by phosphorus accumulation accelerated from 0.2 to 1.3 keq ha-1 yr-1.The national averaged total acidification rates thus increased from 3.3 to 7.9 keq ha-1 yr-1, with an average of 5.6 keq ha-1 yr-1, being approximately 3.0 times as high as the acidification rates of forest soils. The most serious soil acidification occurred in the Jiangsu province with rates greater than 15.0 keq ha-1 yr-1. Regarding the causes of the accelerated acidification: 55.1%was from fertilization, and 6.8%, 34.2% of protons were due to deposition and crop harvest, respectively.The inducement to acidification was increased food demand by population growing, and the direct factor were enormous nitrogen fertilizer application and decreased nitrogen use efficiency.3. Generally, the acid buffering capacity of typical calcareous soils in 0-30cm depth in China was greater than 4000 keq ha-1, which can maintain the soil pH at a relative high level for more than 170 years. However,non-calcareous soils were much sensitive to acid load, since their buffering capacity was only between 286 keq ha-1 and 726 keq ha-1 in same depth. Therefore, acid-sensitive areas in China were in the South and Northeast,where the non-calcareous soil are mainly distributed. Results showed greater acidification rates in double cropping systems (wheat-maize, wheat-rice and rice-rice) than in the single cropping system (single maize). The predicted average pH decline was 0.86 units in single maize system, if the rate of nitrogen fertilizer application continuing increase at 1% per year (BAU scenario). Whereas, the decline in wheat-maize, wheat-rice and rice-rice cropping systems were 2.7,1.7 and 1.4 units, respectively. Al3+ release in soil solutions was predicted in all of these cropping systems except for single maize.4. A further analysis was conducted on the basis of five designed scenarios, results predicted that approximately 13% of the China's croplands may suffer from Al toxicity in 2050 following the BAU scenario.While the scenario of no nitrogen fertilizer increase after 2020 (N2020) can reduce the proton production by 12-22% and the threated areas were still predicted greater than 10% of China's croplands. If the crop residues were completely returned to cropland after 2020 (100%RR), proton production was reduced by 27-79% and the threated areas can reduced to 8.9%. Nevertheless, if 30% of applied chemical nitrogen was replaced by manure (30%MR), which can significant reduce the proton production by 82-111%, none of the considered soils will suffering from Al toxicity in 2050. Moreover, the combination of N2020,100%RR and 30%MR,i.e. INMR scenario, is the most effective strategy on reducing acidification which predicted a slightly acidification recovery. INMR is the most effective strategy on reducing acidification and leads to no Al toxicity in croplands in 2050.5. In this study, the impacts of different scenarios on crop yield were also assessed. The predicted cereal yield losses induced by soil acidification were up to 24.2% in 2050 following the BAU scenario, and the losses can reduce to 15.6%, 16.2%, 4.5% and 2.5% in 2050 in response to scenarios N2020, 100%RR,30%MR and INMR, respectively. Therefore, effectively control of soil acidification by high efficiency nutrient management and soil-crop integrated management being essential prerequisites for meeting the increasing food demand in the future.In conclusion, the main drivers of soil acidification in forest and cropland were deposition and improper fertilization, respectively. If none alleviation strategy is applied in the future, a huge threat to cereal production is predicted. Based on our study, the fundamental approach to manage soil acidification is to reduce N application rate and reduce base cations removal from the soil. As for the current strongly acidified soils, we recommend liming to adjust the soil pH to the level which can support a regular production. In this way, we can successfully manage China's cropland acidification and maintain a sustainable production in the future. |
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