氮肥施用对石灰土土下碳酸盐岩溶解消耗CO2的影响机制

The carbon flux of atmospheric CO2 caused by terrestrial carbonate weathering has a similar magnitude with that of the global forests. However, the dissolution of carbonate rocks by nitric acid due to the overuse of N-fertilizers in agricultural areas will greatly weaken the karst carbon sinks, which proportion may up to 7%-38%. Meanwhile, the soil formed by carbonate weathering have higher pH and base saturation, which would have a stronger buffer action to H+. H+ from soil nitrification may be partially or completely buffered. In view of this, dissolved quantity of nitric acid to carbonate rocks may be overestimated using end element method. However, the research has not yet been carried out. In this project, 15NH4NO3 and NH415NO3 double labeling methods will be used to track the transformation rate and route of nitrogen in lime soils after application of nitrogen fertilizer in a series of field watershed with different degrees of agricultural activities and lime soil pot with different degrees of nitrogen fertilizer. The buffer capacity and threshold of lime soil to nitirc acid from soil nitrification were studied. And The ratio of nitric acid and carbonic acid to carbonate dissolution would be distinguised by the combination of isotope, hydrothermal end element method and lime soil acid buffer capacity. The 13C labeling will be used to track the migration and transformation of organic matter in lime soil under the influence of different degree of fertilization activities and its effect on the CO2 depletion by carbonate dissolution. It is an important guideline for estimating the effect of nitrogen application on the reduction of karst carbon sink precisely.

陆地碳酸盐风化形成的大气CO2净汇（岩溶碳汇）与世界森林碳汇量级相当，但农业地区过量氮肥施用形成的硝酸对碳酸盐岩的溶解会减弱岩溶碳汇效应，其量可达到7%~38%。碳酸盐岩风化形成的石灰土具有较高的pH值及盐基饱和度，对H+有较强的缓冲作用。土壤硝化产酸可能部分或全部被缓冲，以往使用的端元法可能过高估算了硝酸对碳酸盐岩的溶蚀量，但该方面的研究尚未开展。本项目以具有不同程度农业活动的一系列自然流域和不同施氮水平石灰土盆栽模拟为研究对象，采用15NH4NO3和NH415NO3双标记法追踪施入氮肥后石灰土氮转化途径与速率；研究石灰土对土壤硝化产物硝酸的缓冲容量及阈值；结合同位素、水化学端元法及石灰土酸缓冲容量甄别硝酸和碳酸对碳酸盐岩溶蚀的比例；利用13C标记法示踪不同施肥活动影响下石灰土有机质的迁移转化及其对碳酸盐岩溶解消耗CO2的影响，为准确估算氮肥施用对岩溶碳汇的减汇作用提供重要的指导意义。

本项目设置了一系列不同施氮浓度的盆栽模拟实验，结合自然流域地下水的观测，开展了施氮对土壤、土壤-地下水岩溶碳循环的影响、流域尺度氮迁移转化及其参与岩溶碳循环的机制研究，发现以下主要结论：.1.施肥明显提高了自养硝化(ONH4)的速率和有机氮向NH4+的矿化，且 N10提高作用最大，从N10到N70，净NO3-产生量逐渐降低，分别为不施肥的3.66，2.49，2.56和1.57倍。从土壤供氮能力的角度，总体施肥量控制在100 kg N ha-1 a-1左右效果最佳。.2.石灰土存在碳酸溶解碳酸钙、硝酸溶解碳酸钙和阳离子交换三种酸缓冲机制。较低浓度的氮肥（100 kgN ha-1a-1）主要通过增加土壤CO2的浓度间接参与岩溶碳循环，硝化产酸全部由阳离子交换缓冲。在浓度为250~700 kgN ha-1a-1时，45 %的H+直接参与碳酸钙的溶蚀，55 % 的H+被阳离子交换缓冲。100~250 kg N hm-2 a-1施肥量为安全用量，而450~700 kg N hm-2 a-1属于警戒区用量，但尚未达到土壤酸缓冲的阈值。.3.漓江流域地下水同位素的变化范围为-11.97~-16.47‰，比漓江流域地表水偏负。土壤氮和肥料中的NH4+是地下水NO3-的主要来源，有机肥和污水来源较少。地下河中人为输入的NO3占64-92%，土壤输入占8-36%。.4.漓江流域地下水无机碳和钙、镁浓度随NO3-浓度的增加而增加，三者来源于碳酸溶蚀碳酸盐、硝酸溶蚀碳酸盐和阳离子交换三个过程。在人为输入的NO3-＜0.2~0.3 mmol L-1时，以植物充分吸收氮素，刺激微生物呼吸和有机质矿化，增加土壤CO2溶蚀碳酸盐为主；NO3->0.3 mmol L-1，有硝酸溶蚀或阳离子交换。同位素端元法计算地下河硝酸溶蚀碳酸盐的平均值为4.34 %，水化学平衡法计算结果为8.83 %，这4.49 %的差值可能全部为阳离子交换造成。.5.氮肥对土壤CO2的提高作用为10.5 %~30.6 %，土壤呼吸速率提高了7 %~60 %，试片溶蚀速率提高了1.8~3.6倍。施肥导致土壤大气CO2 的源汇量均增加，随施氮量的增加，汇/源比从0.44%上升到0.91%。岩溶流域氮素参与碳循环过程与模式分三阶段：①增加土壤CO2浓度及岩溶表层带碳酸盐的溶蚀；②阳离子交换及硝酸溶蚀土壤碳酸钙；③硝酸溶蚀基岩阶段。