| Increasing organic material input to improve soil organic carbon(SOC)sequestration is an important method to maintain and improve soil fertility,regulate ecosystem carbon cycle as well as mitigate global climate change.SOC sequestration is not only affected by management measures(mineral fertilization,quantity and mode of organic material input,etc.),but also controlled by environmental factors such as water and heat conditions,soil properties,soil nutrient availability,etc.So far,numerous researches have been done on the effect and associated mechanism of organic amendments on soil fertility,especially SOC sequestration.New findings have been reported continuously in revealing the influence factors and driving mechanisms of organic material decomposition,native SOC mineralization and new SOC formation.However,the physical,chemical and microbial mechanisms that dominate SOC sequestration are very complex,and many scientific questions are there need to be further clarified.In practical production,although the combination of organic amendment and mineral fertilization has become routine measures for nutrient management and soil fertility improvement,further perfecting these measuses requires more targeted scientific evidences in specific regions and production systems for achieving the goal of high yield,high efficiency and environmental safety.For the loess plateau's winter wheat production,although many researches have been conducted to study the effect of routine combined use of mineral fertilization and organic amendment on crop production and soil fertility improvement,it is still a lack of long-term situ research which combined the planting and incorporation of green manure(GM)in summer fallow,mineral fertilization,straw return or organic fertilization and aimed at their single or composite effects.Therefore,conducting the regional empirical researches in the field is conducive to revealing the effect of combination of organic amendment and mineral fertilization on crop yield as well as the effects and mechanisms in SOC sequestration,which can be realized to promote the farmland SOC sequestration and soil fertility improvement,and provide a scientific basis for climate change mitigation.It can also suggest new technologies to achieve the goal of high yield-high efficiency-green development in practical production.Given this background,we conducted field experiments in winter wheat production systems in the two sub-regions with great differences in water and heat conditions as well as soil type,of the semi-humid area prone to drought on the Loess Plateau.The effects of legume GM planting and incorporation in fallow period,straw return and manure application on wheat growth,water and fertilizer utilization efficiency and SOC sequestration,under ridge mulching and furrow sowing as well as balanced fertilization were studied in Changwu(Heilu soil,about 1200 m a.s.l.and 2.5 of dryness index)during 2016-2019,which was to mainly reveal the main effects,interactions and possible mechanisms of GM incorporation,organic material incorporations and the combinations of various measures,on the basis of balanced fertilization.In Yangling(Lou soil,about 1200 m a.s.l and 1.5 of dryness index),the effects of continuously multi-year straw mulching(4500 kg hm-2 year-1)and different nitrogen application rates(0,120 and 240 kg hm-2)on wheat yield,soil structure and SOC sequestration were studied which based on a long-term field experiment during 2002-2016.And based on the difference in soil fertility that caused by contrasting nitrogen application rates(0 and 120 kg hm-2)in the prior 14 years,we carried out the research on the effects and mechanisms of organic amendments(straw return,7500 kg hm-2;manure application,30000kg hm-2)and contrasting N application rates(0,120 kg hm-2).The main results are as follows:1.The experiment was carried out under the conditions of balanced NPK fertilization and ridge mulching-furrow sowing that combined legume GM planting and incorporation in summer fallow with organic amendments.The 3 consecutive growing seasons(2016-2017,2017-2018,2018-2019)were all normal flow years(rainfall about 300 mm)and a serious freezing injury occurred in early April,2018.The results revealed:(1)compared with no GM planting,GM planting and incorporation did not significantly improve winter wheat yield and its water use efficiency.Straw return,manure application and straw return+manure application also did not show significant effect on wheat yield increase.(2)In terms of partial factor productivity(PFP)of N,P and K fertilizers,there was no regular effect of GM incorporation and the above organic amendments on the PFP of 3 mineral fertilizers during2016-2017 and 2017-2018.However,during 2018-2019,the PFP of N,P and K fertilizers were 42.8,48.1 and 96.2 kg kg-1 in average,respectively,in the treatment with NPK fertilization but without GM planting.And in the treatment with NPK fertilization and GM incorporation,the PFP of N,P and K fertilizers were 46.0,51.8 and 103.5 kg kg-1 in average,respectively.This indicated that GM planting and incorporation could improve the PFP of 3mineral fertilizers.At the same time,compared with only NPK fertilization,NPK+straw return and NPK+manure application also increased PFP of N,P and K fertilizers.(3)During2018-2019,compared with the no GM planting,GM incorporation increased the N content in wheat grains by 4.4%,and increased apparent utilization rate of N fertilizer by 26.8%,indicating that GM planting and incorporation in summer fallow could effectively improve the wheat nutritional quality and N fertilizer efficiency.Regardless of GM,although NPK+straw return and NPK+manure application did not significantly increase N content in wheat grains compared with only NPK fertilization,they both improved the apparent utilization rate of N and K fertilizers due to the increase in wheat yield.2.Compared with no GM planting,GM planting and incorporation increased SOC stock by 5.6%(P<0.05)in 0-20 cm soil layer over 2016-2017 and 2017-2018,and the apparent sequestration efficiency(CSE)of GM-derived organic C was 27.7%.Compared with only NPK fertilization,three treatments including NPK+straw return,NPK+manure application and NPK+straw return+manure application,increased the SOC stock by 3.8%,18.0%and 23.7%,respectively,when GM was absence,and their CSE were 4.0%,30.8%and 25.1%,respectively.However,when GM was presence,the above 3 treatments increased SOC stock by 10.8%,20.4%and 28.5%,respectively,relative to only NPK fertilization,and their CSE were 15.6%,29.0%and 26.7%,respectively,indicating that planting and incorporation of legume GM in the summer fallow could further promote the SOC sequestration under other organic amendments.In this experiment,no significant interaction was observed when the green manure turning over,straw returning and organic fertilizer application were coupled due to NPK nutrients was supplied sufficiently,thus the net SOC sequestration significantly positively correlated with the cumulative organic C input.3.The results of the field experiment of annual straw mulching and three N application rates from 2002 to 2016 showed that:(1)compared with no straw mulching,straw mulching had no significant effect on wheat yield increase,but significantly increased SOC stock and C management index(CMI)in 0-20 cm soil layer by 5.7%and 13.3%respectively.Compared with no N application,N application at 120 and 240 kg ha-1increased the 14 years'average wheat yield by 47.1%and 49.1%,respectively,with no significant difference between the two N rates(P>0.05).When no straw was mulched,N application at 120 and 240 kg ha-1 increased SOC stock by 4.9%and 9.5%,respectively,with the CMI increased by 20%and 12%.However,with straw mulching,the two N rates increased SOC stock by 7.3%and 7.0%,respectively,with the CMI increased by 20%and6.9%.This was mainly due to that N input increased the wheat biomass which leaded to increased organic residue input,and thus benefited to SOC sequestration.However,excessive N deposition in soil would accelerate mineralization of labile organic C,and reduce CMI.(2)Compared with no straw mulching,straw mulching increased the mass proportion of macro-aggregates by 3.6%on average,while decreased the mass proportions of micro-aggregate and silt+clay by 1.1%and 2.5%,respectively.Compared with no N application,N application at 120 kg ha-1 had no significant effect on the composition of soil aggregates,while 240 kg ha-1 reduced the mass proportion of soil macro-aggregates by 5.5%,and increased the mass proportions of soil microaggregates and silt+clay by 3.2%and 2.3%,respectively.Straw mulching and N application at 120 kg ha-1 could effectively promote SOC sequestration,macro-aggregation and improve the soil CMI.4.The results of organic amendments and N application experiment from 2016 to2019 showed that(1)compared with no organic amendment,straw return and manure application increased the SOC stock in 0-20 cm layer by 11.8%and 30.2%on average(P<0.05),respectively.Specifically,the SOC sequestration rates after straw return in soils that subjected to long-term N absence and N fertilization were 0.74 and 0.92 Mg ha-1 yr-1,respectively,the latter was 24.3%higher than the former(P<0.05),with CSE at 18.5%and18.1%,respectively.This indicates that soil N deficiency can significantly reduce SOC sequestration capacity under straw return,but increase the rate and efficiency of SOC sequestration under manure application(P<0.05).(2)The SOC was divided into four components,including high labile,medium labile,low labile and non-labile using KMn O4solution oxidation method with different concentration gradients.Their contents were in order non-labile>very labile>less labile>labile component.Compared with no organic amendment,straw return increased the contents of above four SOC components by 21.8%,1.2%,12.0%and 12.1%on average,respectively,whereas manure application increased the four components by 48.4%,71.3%,35.2%and 29.2%on average,respectively.In contrast,the differences were small between soils that subjected to long-term contrasting N applications in SOC content across the 4 components(P>0.05).Compared with long-term N absence,the relative increases(relative to control)in both very labile and non-labile components of SOC that subjected to long-term N fertilization were significantly lower(P<0.05).Especially,the relative increase in very labile component of SOC under straw return was lower by 49.5%,while the relative increase in non-labile component of SOC under manure application was lower by 27.2%.This indicates that increased soil N deposition would negatively affect the increase in very labile SOC component after straw return,while negatively affect on the increase in non-labile SOC component after manure application,to some extent.This study summarized the results and conclusions can be made as follows:(1)On the basis of balanced NPK fertilization as well as ridge mulching and furrow sowing,the main contribution of planting and incorporation of legume GM in the summer fallow is to improve the nutritional quality of wheat,partial factor productivity of mineral fertilizer and organic C sequestration capacity in topsoil,especially has the ability to further increase the effect on SOC sequestration of other organic amendments.When abundant macro-element nutrients are supplied in the soil,SOC sequestration rate mainly depends on the cumulative organic C input,and there is no significant interaction on SOC sequestration between GM incorporation,straw return and manure application.(2)N input can increase the organic residue input in the field by increasing crop biomass,thus straw return is expected to be more conducive to SOC sequestration and soil aggregation only when in the soil with sufficient N.(3)The capacity in SOC sequestration of N-deficient soil response differentially to straw return and manure application.Although N-deficient soil would induce a less quantity of plant-derived organic C input,applied farmyard manure with relatively low C/N ratio likely weakened the native SOC mineralization that caused by microbial N mining,reducing the SOC loss and increased the C sequestration efficiency,suggesting that manure application is more effective in SOC sequestration when applied in the N-deficient soil.Our findings can provide field location test data for rain-fed winter wheat production to achieve the goal of high yield-high efficiency-soil fertility improvement-environmental friendliness on the Loess Plateau. |
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