electronic supplementary material

DNRA Calculations

DNRA estimation by M1

Gross DNRA rate (d, mg N kg–1 d–1) was estimated by the recovery of 15NO3– in NH4+ and ON pools (Yin et al. 2002) and was calculated using the following equation:

……… (1)

where [NH4+]t is the soil NH4+–N concentration at time t (mg N kg–1); [NH4+]0 is the soil NH4+–N concentration at the beginning of incubation (mg N kg–1); ftNH4 is the soil NH4+–15N atom% at time t (%); f0NH4 is the soil NH4+–15N atom% at the beginning of incubation (%); [ON]t is the soil ON concentration at time t (mg N kg–1); [ON]0 is the soil ON concentration at the beginning of incubation (mg N kg–1); ftON is the soil ON–15N atom% at time t (%); f0ON is the soil ON–15N atom% at the beginning of incubation (%); and t is the incubation time (d).

DNRA estimation by M2

DRNA rate was calculated using equation 2 (Huygens et al. 2008):

……… (2)

where [15NH4+]avNO3 is the average concentration of soil NH4+–15N at the beginning of incubation and time t in 15NO3– tracing experimental group (mg 15N kg–1); [15NH4+]tNO3 is the soil NH4+–15N concentration at time t in 15NO3– tracing experimental group (mg 15N kg–1) ; [NH4+]0NO3 is the soil NH4+–N concentration at the beginning of incubation in 15NO3– tracing experimental group (mg N kg–1); [%NH4+]0NO3 is the soil NH4+–15N atom% at the beginning of incubation in 15NO3– tracing experimental group (%); [%NO3-]avNO3 is the soil average NO3––15N atom% at the beginning of incubation and time t in 15NO3– tracing experimental group (%); and l is MRT of NH4+ (d) and was determined by the 15NH4+ dilution experimental group:

………………………………………………………………… (3)

where [NH4+]0NH4 is the soil NH4+–N concentration at the beginning of incubation (mg N kg–1); and CA is the gross NH4+ consumption rate (mg N kg–1 d–1) and was calculated by (Hart et al. 1994):

…………………………………………… (4)

where [NH4+]tNH4 is the soil NH4+–N concentration at time t (mg N kg–1); and m is the gross NH4+ production rate (mg N kg–1 d–1) and was calculated as follows (Hart et al. 1994):

……(5)

where APE0 and APEt are the NH4+–15N atom% excesses at the beginning of incubation and time t, respectively (%).

The contribution of DNRA to gross NO3− consumption was calculated as follows:

………………………………………………………………… (6)

where CN is the gross NO3– consumption rate (mg N kg−1 d−1) and was determined by 15NO3– dilution experimental group (Hart et al. 1994):

……………………………………………… (7)

where [NO3−]0NO3 is the soil NO3−–N concentration at the beginning of incubation (mg N kg–1); [NO3−]tNO3 is the soil NO3−–N concentration at time t (mg N kg–1); n is the gross NO3− production rate (mg N kg−1 d−1) and was calculated as follows (Hart et al. 1994):

………(8)

where NPE0 and NPEt are the NO3−–15N atom% excesses at the beginning of incubation and time t, respectively (%).

References Electronic Supplementary Material

Hart SC, Stark JM, Davidson EA, Firestone MK (1994) Nitrogen mineralization, immobilization, and nitrification. In: Weaver RW, Angle S, Bottomley P, Bezdiecek D, Smith S, Tabatabai A, Wollum A, Mickelson SH, Bigham JM (eds) Methods of Soil Analysis (Microbiological and Biochemical Properties, Part 2), Soil Science Society of America, Madison, Wisconsin, pp 985–1018

Huygens D, Boeckx P, Templer PH, Paulino L, Van Cleemput O, Oyarzún CE, Müller C, Godoy R (2008) Mechanisms for retention of bioavailable nitrogen in volcanic rainforest soil. Nat Geosci 1:543–548

Rice CW, Tiedje JM (1989) Regulations of nitrate assimilation by ammonium in soils and in isolated microorganisms. Soil Biol Biochem 21:579–602

Yin SX, Chen D, Chen LM, Edis R (2002) Dissimilatory nitrate reduction to ammonium and responsible microorganisms in two Chinese and Australian paddy soils. Soil Biol Biochem 34:1131–1137