Effects Of Long-term Fertilization On Soil Organic Carbon Fractions In Dry Land Of Loess Plateau

Soil organic carbon?SOC?is the essential component and the largest organic carbon pool in terrestrial ecosystems,it plays many key multifunctional roles in soil fertility.The level of SOC content of farmland soil affects not only soil nutrient supply and crop productivity,but also greenhouse gas emissions and global climate change.Improving soil SOC and meanwhile reducing CO₂ emissions have become an important way to achieve sustainable agricultural development and mitigate the greenhouse effect.SOC sequestration depends largely on the input and mineralization of organic matter,and the efficient sequestration of organic matter is determined by many factors,such as soil properties,characteristics of soil aggregates,organic matter fractions,and so on.The Loess Plateau is the center of dryland agriculture in China.The main cropping system is one crop per year,and double crops per year is only practiced in a small part of the region.In this study,four representative long-term experiments?Yangling,Tianshui,Pingliang,Heyang?on the Loess Plateau were employed to study the crop response to contrasting fertilization regimes in different planting systems,the quantities of organic carbon input to the field,the dynamics of OC and its labile fractions,the efficiency,rate and potential of OC sequestration;the composition and distribution of OM in soil aggregates and silt+clay fractions separated with dry and wet sieving;we also studied the impact of fertilization regimes on organic carbon pools in bulk soils and those protected by soil aggregates isolated with SOM fractionation techniques;we further examined the effects of long-term fertilization on SOC mineralization and temperature sensitivity with or without addition of crop straw through incubation experiments at different temperatures.Treatments involved in the long-term experiments of this study were:Yangling:irrigation experiment?CK,N,NK,PK,NP,NPK,SNPK,M1NPK and M2NPK?,rain-fed experiment?CK,N,NK,PK,NP,NPK and MNPK?;Tianshui:?CK,NP and MNPK?;Pingliang:?CK,NP and MNP?;Heyang:?CK,N1P1,N2P2 and MN2P2?.The main results obtained were listed below:?1?The grain yield showed no significant difference between treatment receiving no fertilizers?CK?and unbalanced fertilization treatments?N,NK and PK?at any circumstances,and the annual mean grain yield for these treatments ranged from 3.3 Mg ha^-1to 4.7 Mg ha^-1?total yield of wheat and maize?and 1.4-2.0 Mg ha^-1,respectively,under irrigated and rain-fed conditions.There were also no significant difference in grain yield between balanced fertilization treatments those receiving both N and P containing fertilizers,such as NP,NPK and synthetic N and P fertilizers plus organic manure?MNPK?,and the mean annual grain yield ranged from 11.8-13.2 Mg ha^-1 and 4.7-4.9 Mg ha^-1,respectively,under irrigated and rain-fed conditions,which were much greater than those of their corresponding unbalanced fertilization treatments.Like the results of crop yield,there was no significant difference in OC input between CK and unbalanced fertilization treatments,which ranged from 21.4-24.4 Mg ha^-1 and 9.8-14.6 Mg ha^-1,respectively,under irrigated and rain-fed conditions.By contrast,the average amount of OC input in treatments those receiving balanced chemical fertilizers were 61.2 Mg ha^-1 and 28.8 Mg ha^-1,respectively;while it was 109.6-162.1 Mg ha^-1and 69.7 Mg ha^-1 in treatments receiving chemical fertilizers in combination with organic supplements under irrigated and rain-fed conditions,respectively.After 25 years of long-term fertilization,compared with CK and unbalanced fertilization treatments,balanced fertilization could increase SOC storage by more than 30%,synthetic fertilizers applied in combination with manure increased by 50-156%.The ranges of soil carbon sequestration rates were 0.37 Mg ha^-1 y^-1to 1.89 Mg ha^-1 y^-1 and-0.06 Mg ha^-1 y^-1to 0.93 Mg ha^-1 y^-1,respectively,for irrigated and dryland soils on a 5 years basis;The mean carbon sequestration efficiency was 19%?14-32%?and 28%?14-32%?,respectively.The particulate organic carbon?POC?contents increased by 4-180%and 112-160%,respectively,under irrigated and rain-fed conditions as compared to their corresponding initial soils.The relationship between relative yield and SOC content suggested that the threshold SOC contents for obtaining the highest crop yield were 9.97 g kg^-1 and 8.77 g kg^-1,respectively,for the winter wheat-summer maize and winter wheat-summer fallow cropping systems.?2?The dry land soil in the Loess Plateau was mainly composed of marcroaggregatess??0.25 mm?,accounting for more than 70%,followed by microaggregates?0.053-0.25 mm?for 13-17%and silt+clay fractions?<0.053 mm?for less than 10%.Compared with CK treatment,MNP/K treatment increased the proportion of macroaggregates by 21%,decreased the proportion of microaggregates and silty clay by 21%and 16%,respectively.Fertilization treatment significant increased MWD by 9.3%relative to CK.The concentrations of SOC and TN showed no significant difference between soil aggregates size,with an average of 10.6 g kg^-1 and 1.08 g kg^-1,respectively,but it was significantly higher than that in silt+clay fractions(7.36 g kg^-1 and 0.75 g kg^-1,respectively).SOC and TN were mainly distributed in macroaggregates?79%and 77%,respectively?,followed by microaggregates?14%and 15%,respectively?,and silt and clay fractions were the lowest?7%and 8%,respectively?.The enrichment factor?E?of SOC and TN were close to 1 in aggregates,which were significantly higher than that in silt and clay fractions.?3?Long-term fertilizers application in the Loess Plateau significant increased SOC and TN storage by 87.5-483.1 kg ha^-1y^-1 and 8.9-52.5 kg kg^-1y^-1.Mineral-associated OC?MOC?and Mineral-associated N?MON?accounted for 45-75%and 64-89%of SOC and TN,respectively,compared to CK,the annual increment of these fractions were 81.6-237.6kg ha^-1 y^-1 and 3.6-31 kg ha^-1 y^-1,respectively.The storages of POC and PON accounted for25-55%and 11-36%of SOC and TN,and annual increments of these fractions were84.9-306.7 kg ha^-1 y^-1 and 3.2-24.2 kg ha^-1 y^-1,respectively,compared to CK treatment.Light-POC and Light-PON accounted for 8-32%and 6-24%of SOC and TN,respectively,over CK,the annual increments of these fractions were 27.2-198.9 kg ha^-1 y^-1 and 2.4-17.0kg ha^-1 y^-1,respectively.Heavy-POC and Heavy-PON accounted for 13-28%and 4-16%of SOC and TN,respectively.Application of synthetic fertilizers had no effect on Heavy-POM storage,while MNP/K increased it by 1.6-107.5 kg ha^-1 y^-1 and 3.4-9.2 kg ha^-1 y^-1,respectively.Long-term application of synthetic fertilizers had no effect on the distribution of organic matter fractions in soil,while MNP/K increased the proportion of POC and PON by 3-19%.Fertilization had no significant effect on the C/N ratio of bulk soil but can reduce the C/N ratio of POM component.?4?The highest concentration of aggregates SOM fraction was fine intra-POM separated from microaggregate?53f?,it contained 3.54 g C kg^-1 aggregates and 0.21 g N kg^-1 aggregates,respectively,on average.Then fine intra-POM separated from small macroaggregate?0.25-2 mm?,contained 2.45 g C kg^-1 aggregates and 0.15 g N kg^-1aggregates,respectively.There was no significant difference between the other fractions,and the average concentrations of C and N were 1.60 g kg^-1 aggregates and 0.07 g kg^-1aggregates.The highest concentrations of aggregates fractions of soil were Mineral-associated OC and Mineral-associated ON,which were 4.01-9.81 g kg^-1 soil and0.39-1.07 g kg^-1 soil,accounting for 68%and 71%of SOC and TN storage,respectively,in bulk soil.Followed by total fine intra-POC and total fine intra-PON,with 1.03-3.08 g kg^-1soil and 0.12-0.43 g kg^-1 soil,accounting for 18%and 19%of SOC and TN storage,respectively.The Free light POC and Free light PON were the lowest,and only account for5%and 3%of SOC and TN storage.Mineral-associated OM concentrations did not increase with increasing OC content,while the other fractions still have a certain potential to sequestrate more C.Long-term application of synthetic fertilizers had no effect on the proportion of Mineral-associated OM,while MNP/K can decrease the proportion of Mineral-associated OC and ON by 11%and 12%.The C/N in total coarse intra-POM was higher than other fractions.?5?The quantities of cumulative C mineralization of soil were in an order of CK(54-246 mg CO₂-C kg^-1)<NP(81-507 mg CO₂-C kg^-1)<MNP/K(122-641 mg CO₂-C kg^-1)among different temperatures?15?,25?,35??,and accounting for 0.65-2.99%,0.81-5.11%and 0.86-4.50%of total SOC,respectively,in bulk soil.For the treatments with straw addition,the amounts of cumulative C mineralization of soil were also in order of CK(476-1691 mg CO₂-C kg^-1)<NP(547-2031 mg CO₂-C kg^-1)<MNP/K(787-2678 mg CO₂-C kg^-1)among different temperatures,and accounting for 4.9-17.4%,4.8-17.8%and5.1-17.1%of total SOC,respectively.The results of two-component model showed that labile C pool?Ca?and stable C pool?Cs?ranged from 6-38 mg CO₂-C kg^-1 and 61-1853 mg CO₂-C kg^-1,respectively,for the treatments without straw addition,among treatments and temperatures.For the treatments with straw addition,there were 30-60 mg CO₂-C kg^-1 and536-8718 mg CO₂-C kg^-1,respectively,for Ca and Cs pools.The temperature sensitivity of soil CO₂ emissions(Q₁₀)was higher in low temperature?15/25??than that in high one?25/35??,while the result for those with straw addition was just opposite.For the treatments without straw addition,MNP/K was 7%higher than CK in Q₁₀?15/25??,Q₁₀?25/35??was the highest in NP and the lowest in CK treatment.For the treatments with straw addition,Q₁₀ was the highest in NP and MNP/K in low and high temperatures,respectively.Straw addition can reduce Q₁₀?15/25??by 42%.Q₁₀ of Cs was 31-399%and7-418%higher than that of Ca,respectively,for the treatments with and without straw additions.In conclusion,long-term fertilization in the Loess Plateau can significantly improve crop yields and SOC storages,promote the formation of soil macroaggregates,and increase aggregates'stability,long-term application of synthetic fertilizers combined with manure can increase the proportion of soil labile OM,thereby improve the quality of SOM.Although fertilization can improve SOM sequestration,there is a threshold value for crop yield with the increase of SOC content,and fertilizer application has the risk of increasing soil CO₂ emissions and its temperature sensitivity.Therefore,it is necessary to consider not only high and stable yields,but also crop yield threshold and environmental benefits.