| Planted forests are the principal source of industrial wood and play an important role in ecological services.The decomposition process of litterfall,the carrier of nutrients in forest ecosystems,is mainly affected by its quality and climate.In arid and semi-arid areas with combined rainfall and high temperature at the same season,precipitation is the main climatic factor affecting litter decomposition.However,it is still disputed how changing precipitation affects litter decomposition and the relationship between substrate quality,soil nutrient and litter mass loss during litter decomposition.In particular,there is a lack of research on the effect of precipitation-related alternate soil drying and wetting on litter decomposition.In order to better understand the process of mass loss,the dynamic of substrate quality and soil nutrients in relation to leaf-litter mass loss,I studied leaf-litter decomposition of two major planted tree species,i.e.,black locust(Robinia pseudoacacia)and Chinese red pine(Pinus tabulaeformis)in the loess hilly region with litter bag method in the field.Simulation experiments were also used with indoor cultivation(i.e.,designment of a constant humidity,10-day,15-day,and 20-day drying and wetting alternations)in order to study the effect of drying and wetting alternation on the leaf-litter decomposition of black locust and Chinese red pine.The results were as follows.(1)Decomposition time,litter type and their interactions significantly influenced the mass loss and nutrient dynamic in leaf-litter decomposition.During decomposition,the mass loss rate of leaf litter of the two study tree species showed obvious seasonal characteristics,and was the highest in summer rainy season and the lowest in the winter,respectively.The Olson decomposition model could better explain the leaf-litter decomposition of black locust and Chinese red pine.According to this model,the decomposition rate(k value)of leaf litter of black locust was 1.50 times higher than Chinese red pine.The time of 95% mass loss for black locust and Chinese red pine would require 6.65 yr and 9.82 yr,respectively.During the 391-day decomposition,the leaf-litter C,N,and K contents of the two study tree species showed a pattern of net release,and the release rate of black locust was higher than that of Chinese red pine.And the release rate of C,N and K was K> N> C for the same tree species.But leaf-litter P showed a pattern with alternate enrichment and release,and the P enrichment rate of black locust was higher than that of Chinese red pine.However,the dynamic of leaf-litter stoichiometric ratios were significantly different between black locust and Chinese red pine.During the leaf-litter decomposition of black locust,the C/N ratio did not change significantly.In contrast,the ratios of C/P and N/P revealed a trend of decline.The N/P ratio of Chinese red pine did not change significantly,but both C/N and C/P ratios showed a fluctuated upward trend.(2)The addition of leaf litter did not significantly affect soil AK(available potassium)and the ratios of C/P,C/K and N/K for black locust.And adding leaf litter did not significantly affect soil TP,AN(available nitrogen),AK,and the ratios of C/N,C/K,N/K,AN/AP and AN/AK for Chinese red pine.Besides,decomposition time had no significant effect on the soil TN and C/P ratio of black locust.During the leaf-litter decomposition over time,the soil OC and AP contents and C/P ratio of black locust showed an increased trend with undulation.The trend of TN rose at beginning and then decreased(peak in the late rainy season).AK content was stable from 0 to140 days,increased from 141 to 315 days and decreasedd from 315 to 391 days.The dynamic of C/N ratio showed a V shape and N/P ratio displayed an opposite shape to C/N ratio.The ratios of C/K,N/K and P/K showed a downward trend of undulation.The dynamic of soil OC and TN of Chinese red pine showed a U trend,while the dynamic of AP,AK and C/N ratio showed inverted V trends(all peaked in the late rainy season).The dynamic of both C/P and N/P ratios were opposite to C/N ratio.Both the dynamic of C/K and N/K ratios showed V shape.P/K ratio showed a decline trend.Meanwhile,the dynamic of soil TK content both in two study tree species displayed an increasing pattern during leaf-litter decomposition and TP content showed a trend that decreased first and then increased during the 315 day decomposition.(3)During leaf-litter decomposition,the mass loss of leaf litter was different in relation to the dynamic of leaf-litter substrate quality and soil nutrient for both black locust and Chinese red pine.The leaf-litter mass loss of black locust was positively correlated with its C/K,N/K and P/K ratios of leaf litter,but negatively correlated with its C,N,K,and ratios of C/P and N/P of leaf litter,and quadratic function with its P content of leaf litter.But the leaf-litter mass loss of Chinese red pine was negatively correlated the dynamic of its TK content of leaf litter,positively correlated with its C/K,N/K and P/K ratios of leaf litter,and quadratic function with its C/N,C/P and N/P ratios of leaf litter.In addition,there was significant relationship between the mass loss rate of leaf litter and the dynamics of soil nutrients in undergrowth.The leaf-litter mass loss of black locust showed quadratic function with its soil OC and AP,was positively correlated with its soil TK content and ratios of N/K AN/AP and AN/AK and negatively correlated with soil AN content,C/K and P/K ratio.However,the leaf-litter mass loss of Chinese red pine was negatively correlated with its soil OC,AN,and ratios of C/N,C/K and P/K,and positively correlated dynamically with soil TK,AK,and ratios of AN/AP,AP/AK and AN/AK.(4)The mass loss of leaf litter was significantly affected by the type of leaf litter,the time of decomposition,the treatment of alternate drying and wetting and transplanting habitats.However,except for the interaction between leaf-litter type and decomposition time,the interaction of other factors has no significant effect on the mass loss of leaf litter.During the 180-day decomposition,the leaf-litter mass loss for the two study tree species both showed an increasing trend during decomposition over time.Besides,the leaf-litter mass loss under different alternate drying and weting treatments from high to low was: a constant humidity,10-day,15-day,and 20-day drying and wetting alternations.The analysis of leaf-litter mass loss of in-situ and ex-situ decomposition under different treatment of wetting and drying found that,there was no significant difference in leaf-litter mass loss of black locust of in-situ decomposition(soil of black locust)and ex-situ decomposition(soil of Chinese red pine)under constant humidity condition,but no significant difference in the leaf-litter mass loss of Chinese red pine of in-situ and ex-situ decompositon under 20-day drying and wetting alternation.These results indicated that under the influence of precipitation,the decomposition of leaf litter in loess hilly area showed obvious seasonal patterns with peak observed in summer rainy season.The release rate of carbon,nitrogen,potassium and the enrichment rate of phosphorus in high-quality leaf litter(black locust)is higher than that of low-quality leaf litter(Chinese red pine).Leaf-litter decomposition of both tree species promoted the continuous accumulation of TK and the continuous release of AN in soil.However,the relationship between leaf-litter mass loss and nutrient dynamics of leaf litter and soil was greatly influenced by tree species.And the leaf-litter decomposition of black locust was mainly restricted by leaf-litter P content in initial decomposition stage and restricted by leaf-litter N content after initial decomposition,while the leaf litter of Chinese red pine was mainly restricted by leaf-litter N content during the whole decomposition stage.But the leaf-litter mass loss of both tree species was significantly affected by the ratios of soil OC,TN and TP to TK and soil available nutrients and their ratios.The soil sufficient moisture and the increased drought intensity respectively weakened the home-field advantage of leaf-litter decomposition of black locust and Chinese red pine under alternate soil drying and weting.The results of this study might help for us to better understand the nutrient cycling mechanism of forests ecosystem in loess hilly region under the change of precipitation pattern,thus providing more scientific guidance for the management of planted forests in this area. |
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