Definition: | Approximately 250 g of soil (oven dry weight equivalent (ODW)) was placed in 487-ml gas-tight vessels and pre-incubated under either oxic or anoxic conditions for 10 days (n = 4 per treatment; total n = 64). Bulk oxic and anoxic conditions were created by flushing the jar headspace with either CO2-free air (oxic treatment) or N2 gas (anoxic treatment) (Pett-Ridge and Firestone 2005, Hall et al. 2015); jars were sealed and maintained under these headspace conditions for 10 days either in ambient air (oxic) or inside a glove box (anoxic) in order to reach oxic or anoxic conditions before the addition of P and C substrates. Following the pre-incubation period, the soils were amended with one of 3 different solutions, varying in their C:P ratios on a molar basis (7:1, 60:1 and 200:1) (Cleveland and Liptzin 2007, Wright et al. 2011). The C was added in the form of glucose in a concentration of 100 µg C g soil-1 (ODW), and P was added as potassium phosphate (KH2PO4) to achieve the desired C:P ratios. The substrate solutions were supplemented with N in the form of ammonium nitrate to keep the soil C:N ratio close to value of the untreated soil (13:1, Table S1, similar to the mean value in tropical forest soils (Cleveland and Liptzin 2007, Xu et al. 2013)). One set of samples received deionized water only and served as a control. The C:P ratio of soil organic matter (SOM) in the control samples was 240:1 (see supplementary method), within the range of the C:P ratio of SOM in humid tropical forests (Cleveland and Liptzin 2007, Xu et al. 2013). The glucose was labeled with 13C (99% 13C6-glucose, Sigma Aldrich ltd) and used to assess microbial cell uptake and microbial CUE. The total volume of the substrate or control solutions added to the soil was 10 mL; this had a negligible effect on the natural water content of the 250 g soil in each vessel. | Microbial biomass C:P ratios are expressed on a molar basis. | Standard deviation of the carbon to phosphorus ratio for the treatment. | The Sample number | Dissolved organic carbon (DOC) is the fraction of total organic carbon operationally defined as that which can pass through a filter size that typically ranges in size from 0.22 and 0.7 micrometers.[1] The fraction remaining on the filter is called particulate organic carbon (POC). | standard deviation of DOC PPM | DOC+ (ug C g soil-1) | stdev of DOC+ (ug C g soil-1) | DOC- (ppm) | stdev DOC- (ppm) | DOC- (ug C g soil-1) | stdev DOC- (ug C g soil-1) | For microbial biomass C determinations, 10 g of soil (ODW) was fumigated with chloroform for 72 h and extracted with 0.5 M of potassium sulfate (K2SO4) for 1 h. Microbial C was determined as the difference between fumigated and unfumigated samples, and corrected for unrecovered biomass using a conversion factor of 0.45 (Vance et al. 1987). | Because the high contents of amorphous Fe and Al minerals likely contributed to strong P sorption, we adopted the method outlined by Kouno et al (1995) to measure microbial biomass P. Approximately 5 g of soil (ODW) were shaken for 24 h with 80 mL deionized water in the presence of 5 strips of anion exchange resin membrane (BDH-55164, VWR International, Lutterworth, UK) and fumigated with 1 mL hexanol. The P was eluted from the resin membranes by shaking for 24 h with 0.25 M HNO3, and the recovered P was determined by molybdate colorimetry in duplicate with an average difference between duplicates of 1.3%. We estimated the proportion of P sorbed to soil minerals by spiking the extraction system (i.e., soil + anion exchange membrane + water) with 50 µg P g-1 soil and found that an average of 35% of the added P was recovered by anion exchange membrane. Microbial biomass P was calculated as the difference between fumigated and unfumigated samples using a Kp factor of 0.4 to correct for fumigation efficiency in releasing P (Brookes et al. 1982) and the recovery rate against sorption we measured here. | Microbial biomass C:P ratios are expressed on a molar basis. |
Measurement Values Domain: | Definition | Approximately 250 g of soil (oven dry weight equivalent (ODW)) was placed in 487-ml gas-tight vessels and pre-incubated under either oxic or anoxic conditions for 10 days (n = 4 per treatment; total n = 64). Bulk oxic and anoxic conditions were created by flushing the jar headspace with either CO2-free air (oxic treatment) or N2 gas (anoxic treatment) (Pett-Ridge and Firestone 2005, Hall et al. 2015); jars were sealed and maintained under these headspace conditions for 10 days either in ambient air (oxic) or inside a glove box (anoxic) in order to reach oxic or anoxic conditions before the addition of P and C substrates. Following the pre-incubation period, the soils were amended with one of 3 different solutions, varying in their C:P ratios on a molar basis (7:1, 60:1 and 200:1) (Cleveland and Liptzin 2007, Wright et al. 2011). The C was added in the form of glucose in a concentration of 100 µg C g soil-1 (ODW), and P was added as potassium phosphate (KH2PO4) to achieve the desired C:P ratios. The substrate solutions were supplemented with N in the form of ammonium nitrate to keep the soil C:N ratio close to value of the untreated soil (13:1, Table S1, similar to the mean value in tropical forest soils (Cleveland and Liptzin 2007, Xu et al. 2013)). One set of samples received deionized water only and served as a control. The C:P ratio of soil organic matter (SOM) in the control samples was 240:1 (see supplementary method), within the range of the C:P ratio of SOM in humid tropical forests (Cleveland and Liptzin 2007, Xu et al. 2013). The glucose was labeled with 13C (99% 13C6-glucose, Sigma Aldrich ltd) and used to assess microbial cell uptake and microbial CUE. The total volume of the substrate or control solutions added to the soil was 10 mL; this had a negligible effect on the natural water content of the 250 g soil in each vessel. |
| Definition | Microbial biomass C:P ratios are expressed on a molar basis. |
| Definition | Standard deviation of the carbon to phosphorus ratio for the treatment. |
| Definition | The Sample number |
| Definition | Dissolved organic carbon (DOC) is the fraction of total organic carbon operationally defined as that which can pass through a filter size that typically ranges in size from 0.22 and 0.7 micrometers.[1] The fraction remaining on the filter is called particulate organic carbon (POC). |
| Definition | standard deviation of DOC PPM |
| Definition | DOC+ (ug C g soil-1) |
| Definition | stdev of DOC+ (ug C g soil-1) |
| | Definition | stdev DOC- (ppm) |
| Definition | DOC- (ug C g soil-1) |
| Definition | stdev DOC- (ug C g soil-1) |
| Definition | For microbial biomass C determinations, 10 g of soil (ODW) was fumigated with chloroform for 72 h and extracted with 0.5 M of potassium sulfate (K2SO4) for 1 h. Microbial C was determined as the difference between fumigated and unfumigated samples, and corrected for unrecovered biomass using a conversion factor of 0.45 (Vance et al. 1987). |
| Definition | Because the high contents of amorphous Fe and Al minerals likely contributed to strong P sorption, we adopted the method outlined by Kouno et al (1995) to measure microbial biomass P. Approximately 5 g of soil (ODW) were shaken for 24 h with 80 mL deionized water in the presence of 5 strips of anion exchange resin membrane (BDH-55164, VWR International, Lutterworth, UK) and fumigated with 1 mL hexanol. The P was eluted from the resin membranes by shaking for 24 h with 0.25 M HNO3, and the recovered P was determined by molybdate colorimetry in duplicate with an average difference between duplicates of 1.3%. We estimated the proportion of P sorbed to soil minerals by spiking the extraction system (i.e., soil + anion exchange membrane + water) with 50 µg P g-1 soil and found that an average of 35% of the added P was recovered by anion exchange membrane. Microbial biomass P was calculated as the difference between fumigated and unfumigated samples using a Kp factor of 0.4 to correct for fumigation efficiency in releasing P (Brookes et al. 1982) and the recovery rate against sorption we measured here. |
| Definition | Microbial biomass C:P ratios are expressed on a molar basis. |
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