Knowledge of the stability of soil organic C (SOC) is vital for assessing SOC dynamics and cycling in agroecosystems.  Studies have documented the regulatory effect of fertilization on SOC stability in bulk soils. However, how fertilization alters organic C stability at the aggregate scale in agroecosystems remains largely unclear.  This study aimed to appraise the changes of organic C stability within soil aggregates after eight years of fertilization (chemical vs. organic fertilization) in a greenhouse vegetable field in Tianjin, China.  Changes in the stability of organic C in soil aggregates were evaluated by four methods, i.e., the modified Walkley-Black method (chemical method), ¹³C NMR spectroscopy (spectroscopic method), extracellular enzyme assay (biological method), and thermogravimetric analysis (thermogravimetric method).  The aggregates were isolated and separated by a wet-sieving method into four fractions: large macroaggregates
(>2 mm), small macroaggregates (0.25–2 mm), microaggregates (0.053–0.25 mm), and silt/clay fractions (<0.053 mm).  The results showed that organic amendments increased the organic C content and reduced the chemical, spectroscopic, thermogravimetric, and biological stability of organic C within soil aggregates relative to chemical fertilization alone.  Within soil aggregates, the content of organic C was the highest in microaggregates and decreased in the order microaggregates>macroaggregates>silt/clay fractions.  Meanwhile, organic C spectroscopic, thermogravimetric, and biological stability were the highest in silt/clay fractions, followed by macroaggregates and microaggregates.  Moreover, the modified Walkley-Black method was not suitable for interpreting organic C stability at the aggregate scale due to the weak correlation between organic C chemical properties and other stability characteristics within the soil aggregates.  These findings provide scientific insights at the aggregate scale into the changes of organic C properties under fertilization in greenhouse vegetable fields in China.

Soil aggregation, microbial community, and functions (i.e., extracellular enzyme activities; EEAs) are critical factors affecting soil C dynamics and nutrient cycling.  We assessed soil aggregate distribution, stability, nutrients, and microbial characteristics within >2, 0.25–2, 0.053–0.25, and <0.053 mm aggregates, based on an eight-year field experiment in a greenhouse vegetable field in China.  The field experiment includes four treatments: 100% N fertilizer (CF), 50% substitution of N fertilizer with manure (M), straw (S), and manure plus straw (MS).  The amounts of nutrient (N, P₂O₅, and K₂O) input were equal in each treatment.  Results showed higher values of mean weight diameter in organic-amended soils (M, MS, and S, 2.43–2.97) vs. CF-amended soils (1.99).  Relative to CF treatment, organic amendments had positive effects on nutrient (i.e., available N, P, and soil organic C (SOC)) conditions, microbial (e.g., bacterial and fungal) growth, and EEAs in the >0.053 mm aggregates, but not in the <0.053 mm aggregates.  The 0.25–0.053 mm aggregates exhibited better nutrient conditions and hydrolytic activity, while the <0.053 mm aggregates had poor nutrient conditions and higher oxidative activity among aggregates, per SOC, available N, available P, and a series of enzyme activities.  These results indicated that the 0.25–0.053 mm (<0.053 mm) aggregates provide suitable microhabitats for hydrolytic (oxidative) activity.  Interestingly, we found that hydrolytic and oxidative activities were mainly impacted by fertilization (58.5%, P<0.01) and aggregate fractions (50.5%, P<0.01), respectively.  The hydrolytic and oxidative activities were significantly (P<0.01) associated with nutrients (SOC and available N) and pH, electrical conductivity, respectively.  Furthermore, SOC, available N, and available P closely (P<0.05) affected microbial communities within >0.25, 0.25–0.053, and <0.053 mm aggregates, respectively.  These findings provide several insights into microbial characteristics within aggregates under different fertilization modes in the greenhouse vegetable production system in China.

Greenhouse vegetable production has been characterized by high agricultural inputs, high temperatures, and high cropping indexes.  As an intensive form of agriculture, nutrient cycling induced by microbial activities in the greenhouses is relatively different from open fields in the same region.  However, the responses of soil microbial biomass carbon (MBC) and nitrogen (MBN), enzyme activities, microbial community composition, and yield to organic amendment are not well understood.  Therefore, a 5-year greenhouse tomato (Solanum lycopersicum Mill.)-cucumber (Cucumis sativus L.) rotation experiment was conducted.  The field experiment comprised 5 treatments: 4/4CN (CN, nitrogen in chemical fertilizer), 3/4CN+1/4MN (MN, nitrogen in pig manure), 2/4CN+2/4MN, 2/4CN+1/4 MN+1/4 SN (SN, nitrogen in corn straw) and 2/4CN+2/4SN.  The amounts of nitrogen (N), phosphorus (P₂O₅), and potassium (K₂O) were equal in the five treatments.  Starting with the fourth growing season, the optimal yield was obtained from soil treated with straw.  MBC, MBN, phospholipid fatty acid (PLFA) profiles, and enzyme activities were significantly changed by 5 years of substitution with organic amendments.  Redundancy analysis showed that MBC accounts for 89.5 and 52.3% of the total enzyme activity and total community variability, respectively.  The activities of phosphomonoesterase, N-acetyl-glucosaminidase, and urease, and the relative abundances of fungi, actinomycetes, and Gram-negative bacteria were significantly and positively related to vegetable yields.  Considering the effects of organic amendments on soil microbial characteristics and vegetable yield, 2/4CN+1/4MN+1/4SN can improve soil quality and maintain sustainable high yield in greenhouse vegetable production.