Soil Aggregate Stability and Aggregate-Associated Carbon Under Different Tillage Systems in the North China Plain

The inlfuences of tillage systems on soil carbon （C） stocks have been studied extensively, but the distribution of soil C within aggregate fractions is not well understood. The objective of this study was to determine the inlfuences of various tillage systems on soil aggregation and aggregate-associated C under wheat （Triticum aestivum L.） and corn （Zea mays L.） double cropping systems in the North China Plain. The experiment was established in 2001, including four treatments：moldboard plow （MP） with residue （MP＋R） and without residue （MP-R）, rotary tillage with residue （RT）, and no-till with residue （NT）. In 2007 soil samples were collected from the 0-5, 5-10, and 10-20 cm depths, and were separated into four aggregate-size classes （〉2 000, 250-2 000, 53-250, and〈53 μm） by wet-sieving method. Aggregate-associated C was determined, and the relationships between total soil C concentration and aggregation-size fractions were examined. The results showed that NT and RT treatments signiifcantly increased the proportion of macroaggregate fractions （〉2 000 and 250-2 000 μm） compared with the MP-R and MP＋R treatments. Averaged across all depths, mean weight diameters of aggregates （MWD） in NT and RT were 47 and 20% higher than that in MP＋R. The concentration of bulk soil organic C was positively correlated with MWD （r=0.98; P=0.024） and macroaggregate fraction （r=0.96; P=0.036） in the 0-5 cm depth. In the 0-20 cm depth, comparing with MP＋R, total C occluded in the〉2 000 μm fraction was increased by 9 and 6%under NT and RT, respectively. We conclude that adoption of conservation tillage system, especially no-till, can increase soil macro-aggregation and total C accumulation in macroaggregates, which may improve soil C sequestration in the intensive agricultural region of the North China Plain.

Validation of degree-day models for predict!ng the emergence of two fruit flies (Diptera: lephritidae) in northeast Egypt

We estimated thermal developmental thresholds(7o)and degree-day(DD)constants for the immature stages of two tephritid pests,Bcictrocera zonata(Saunders)and Ceratitis capitata(Weidenmann).Males of both species were trapped in an Egyptian guava orchard during the fruiting seasons of 2016 and 2017 and trap catches were compared with peak flights predicted by the DD model based on local weather data.Ceratitis capitata had faster development than B.zonata at 20 and 25°C,but their overall developmental rate was similar at 30 and 35°C.The thermal threshold of development(7o)of B.zonata was higher than that of C.capitata,indicating greater sensitivity to cold.Although 35°C yielded the fastest development of both species,survival was higher at 30°C,with B.zonata experiencing a slight advantage,suggesting better tropical adaptation.Immature development of B.zonata and C.capitata was estimated to require 338 and 373 d,respectively,and 616 and 424 DD for a complete generation.Trap catches over both seasons showed good correspondence to peaks of fly activity predicted by the DD models;deviations from expectation ranged from 0 to 7 d for both fly species.Both species had four overlapping generations per season,with B.zonata abundance peaking in the first generation in both years,but only in 2016 for C.capitata.The models predict about eight and 12 generations per year in northeast Egypt for B.zonata and C.capitata,respectively.These models should be useful for timing pest control measures to coincide with periods of peak fly activity in fruit orchards.