Soluble Organic Nitrogen Losses Associated with Recovery of Mineralized Nitrogen1

S. J. SMITH2

ABSTRACTThe extent and subsequent availability of soluble organic N in soil

mineralization extracts were determined using initially air-dry soilsamples. Involved were eight Southern Plains agricultural soils com-prising five soil orders. The major losses of soluble organic N oc-curred during the initial leaching prior to incubation, and are at-tributed to factors not directly associated with the mineralizationprocedure (e.g., freshly decomposed plant residues, pretreatments,etc.). Often, these losses were as high or higher than initial valuesfor inorganic N. Thereafter, the organic N losses represented, onaverage, only about 5% or less of the inorganic N produced in surfacesoil samples during 84 d of aerobic mineralization. This was thecase for both indigenous and fertilizer-incorporated N. Subjectingthe soluble organic N in the mineralization extracts to subsequentaerobic, anaerobic, or autoclave procedures indicated no excessiveN availability. Distribution of the soluble organic N components inthe extracts did not differ greatly from that of soil. In general, sol-uble organic N losses were not a major factor associated with re-covery of mineralized N.

Additional Index Words: aerobic mineralization, anaerobic min-eralization, I5N, autoclave-distillable N, organic N forms.

Smith, S.J. 1987. Soluble organic nitrogen losses associated withrecovery of mineralized nitrogen. Soil Sci. Soc. Am. J. 51:1191-1194.

THE NEED FOR ACCURATE LABORATORY MEASURESof soil N availability is well recognized. Various

biological and chemical indexes exist (Bremner, 1965;Keeney, 1982; Stanford, 1982), but none is withoutcriticism. Probably the laboratory approach that mostclosely simulates a field situation is one involvingaerobic N mineralization. Typically, a soil sample isplaced in a filter tube, moistened, incubated, and pe-riodically the mineralized N is removed by leaching.A major criticism of this approach, however, is thefact that some soluble organic N may also be removedduring the leaching process (Beauchamp et al, 1986;Bremner, 1965; Broadbent and Nakashima, 1971; Legget al., 1971; Smith et al., 1980). Few actual determi-nations of soluble organic N in the leachate from min-

1 Contribution from the Water Quality and Watershed Res. Lab.,Southern Plains Area, USDA-ARS, P. O. Box 1430, Durant, OK74702. Received 12 Jan. 1987.2 Soil Scientist.

eralized soil systems have been made, but recent re-ported amounts have varied from <0.1 (Beauchampet al., 1986) to >0.5 (Smith et al., 1980) of the totalN leached. That organic N removed is considered torepresent a form that may be readily susceptible tomineralization. To date, the general extent and sub-sequent availability of the soluble organic N has notbeen established. This subject is treated in the presentinvestigation, where soluble organic N losses are con-sidered for a range of soils.

MATERIALS AND METHODSEight surface soils (0-150 mm depth) were available from

a prior field lysimeter study (Smith et al, 1982) involvingthe disposition of 15N-tagged fertilizer associated withsorghum-sudangrass [Sorghum sudanense (Piper) Stapfj pro-duction. All the soil in each lysimeter was collected in thefall after harvest, spread into thin layers to air-dry, and stub-ble and roots were removed by hand. Thereupon, each soilwas crushed gently to pass a 2-mm sieve, mixed thoroughly,and stored in tightly sealed glass containers to await chem-ical and biological analysis. Subsequent procedures wereconducted in duplicate.

Total soil organic N (TKN) was determined by a semi-micro-Kjeldahl procedure (Bremner and Mulvaney, 1982),organic C by wet oxidation (Mebius, 1960), and pH by theglass electrode (soil weight/water volume, 1:2). Aerobic soilN mineralization was conducted at 35 °C using a successiveincubation/leaching procedure (Stanford and Smith, 1972).The study here utilized 20 g of soil and 2 g of exfoliatedvermiculite mixtures in 50-mL glass filter tubes. Each leach-ing involved 50 mL of 0.01 M CaC|2 followed by 25-mLminus-N nutrient solution. The vermiculite, ground to passa 0.85-mm sieve, makes for a uniform, dry mix with soilthat does not tend to segregate upon transfer to the filtertube. Prior testing of the vermiculite used indicated it wouldpose no NH^ fixation problems.

Total soluble organic N in the mineralization extracts wasdetermined using a semiautomated Kjeldahl procedure de-veloped for determining biologically-derived N in waters andwastes (USEPA, 1979). Aerobic N mineralization involvingthe soluble organic N extracts was conducted by bubblingair (2 bubbles s~') through 20-mL samples in enclosed cen-trifuge tubes. Anaerobic N mineralization of the soluble or-ganic N extracts was conducted in a similar manner, butwithout passage of air. Autoclave-distillable N (Smith andStanford, 1971) in the extracts was determined using 20-mLsamples. Organic N fractionation of the extracts was madeby hydrolyzing 5-mL samples (evaporated down from 20

1192 SOIL SCI. SOC. AM. J., VOL. 51, 1987

Table 1. Characteristics of surface soils (0-150 mm depth) used in soluble organic N loss study.

TKN

Soil type

Bowie loamClaremore clayMuskogee loamHuston sandy loamHuston sandy clay loamSan Saba clayTeller loamYahola silty clay

Classification

Fragic PaleudultsLithic ArgiudollsAquic PaleudalfsTypic PaleudultsTypic PaleudultsUdic PellustertsUdic ArgiustollsTypic Ustifluvents

PH

5.76.67.26.26.27.77.47.8

OrganicC

7600170001300080009400

250006800

18000

In-digenous

5291364

830618653

1902666

1720

Pert.Incorp.t

5.612.510.54.48.4

13.65.5

22.1

Initial mineral NJ

In-digenous

5.110.619.49.65.0

12.85.6

63.9

Pert.Incorp.

0.052.031.110.260.072.350.02

32.00

Mineralized N (84 d)§

In-digenous

59.5119

70.476.775.0

14855.7

159

Fert.Incorp.

1.43.22.01.62.13.41.21.1

t Residual 15N-enriched fertilizer incorporation.t Includes any residual "N-enriched fertilizer N present in the original Kls NO, form.

§ Mineralized N was predominantly in the NOj form.

mL) with HC1, utilizing procedures described by Porter etal. (1964).

Inorganic N (NOj, NO;r, and NH^) in the mineralizationextracts was determined by microdiffusion (Stanford et al.,1973) with (for all three N forms) and without (for justNHJ) Devarda's alloy.

Isotope ratio analysis was carried out by the method ofSmith et al. (1963) using a Perkin Elmer RMS4 single-beammass spectrometer.3 Precision of the determinations was ap-proximately ±0.003 atom% I5N.

RESULTS AND DISCUSSIONGeneral Soil and N Characteristics

Pertinent characteristics for the eight surface soilsare summarized in Table 1. Five soil orders are en-compassed, representing a wide range of chemical,physical, and biological properties. Indigenous andfertilizer-incorporated TKN contents ranged from 529to 1902, and 4.4 to 22.1 mg N kg~' soil, respectively.Overall, about 9% of the indigenous TKN was min-

3 Trade names are mentioned for the benefit of the reader andimply no endorsement by the USDA.

Table 2. Soluble organic N in mineralization extracts.

Mineralization period (d)

Soil

Bowie 1Claremore cMuskogee 1Huston siHuston sclSan Saba cTeller'1Yahola sicl

X4

Initial!

158957376

18886

17319

10858

0-14

5.20.94.96.88.62.96.05.25.12.3

14-28 28-56

Indigenous8.22.11.93.22.93.92.86.23.92.2

N, %$6.24.06.89.79.64.05.53.96.22.4

Fertilizer-incorporatedBowie 1Claremore cMuskogee 1Huston siHuston sclSan Saba cTeller 1Yahola sicl

Xsi

457284263

56736

3454

193227

1.50.54.15.52.30.41.30.62.01.9

1.20.20.63.20.42.60.42.31.41.2

0.80.81.35.01.80.61.00.71.51.5

'56-84

i4.92.97.72.47.71.53.58.54.92.7

N, %0.40.20.93.01.20.10.51.41.01.0

0-84

5.92.25.27.55.93.24.95.65.01.7

1.30.42.24.61.90.60.91.01.61.4

t Represents soluble organic N removed prior to incubation.i Represents single standard deviations of the means.§ Values are expressed as a percent of mineralized N for the indicated period.

eralized during 84 d, compared with 23% of the fer-tilizer-incorporated TKN. The percentage for the in-digenous TKN is similar to literature data for theincubation period involved (Smith et al., 1980; Stan-ford and Smith, 1972). Moreover, the correspondingfertilizer-incorporated percentage is consistent withprevious observations that recently incorporated, re-sidual fertilizer organic N forms are much more sus-ceptible to mineralization than indigenous soil N(Smith et al., 1978). Accordingly, a relatively higherloss of soluble organic N might be anticipated fromthe fertilizer-incorporated N.

Organic N in Mineralization ExtractsSoluble organic N in the extracts, expressed as a

percentage of inorganic N produced during incuba-tion, is given in Table 2. Data are delineated accordingto indigenous soil N and fertilizer-incorporated N. Inboth cases, the portion of soluble organic N removedrelative to inorganic N was highest during the initialleaching prior to incubation.

In fact, initial soluble organic N values were oftenas high or higher than corresponding inorganic N val-ues. Most likely, this situation reflects the presence offreshly decomposed plant materials and some re-moval of organic N components degraded or dis-rupted during sample preparation prior to conductingthe N mineralization study. Relative to indigenous soilN, only in certain cases were higher portions of solubleorganic N removed from the fertilizer-incorporated N.

Once the soluble organic N initially present in thesoils was removed, soil amounts solublized subse-quently decreased markedly, comprising on average,only about 5 and 2% of the 84-d mineralized N forindigenous and fertilizer-incorporated N, respectively.The lower value for the latter is attributed to removalof more readily soluble organic N components duringthe initial leaching prior to incubation. On the aver-age, for both indigenous and fertilizer-incorporated N,the portion of soluble organic N removed during eachincubation period remained fairly stable indicating nosequential and/or preferential solubilization of resid-ual organic components during incubation.

In general, with this set of soils and for both indig-enous and fertilizer-incorporated N, only smallamounts of soluble organic N were removed in con-junction with the actual mineralization process. In-stead, most removal was associated with the initialleaching prior to incubation. It should be noted that

SMITH: SOLUBLE ORGANIC LOSSES ASSOCIATED WITH RECOVERY OF MINERALIZED N 1193

Table 3. Mineral Nt produced by incubating or autoclaving extracts from soil N mineralization systems.

Initial extracts

Soil

Bowie 1Claremore cMuskogee 1Ruston siRuston sclSan SabaTeller 1Yahola sicl

Solubleorganic N

7.910.813.49.69.5

12.59.8

15.4

Aerobic

0.5-1.9-2.7nil-0.6-3.0-1.9

-36.9

Anaerobic

1.20.42.00.1nilnilnil0.1

Autoclave

2.60.83.82.21.91.10.32.4

Extracts from 14-d soil incubations

Solubleorganic N

1.50.51.42.42.71.81.53.4

Aerobic

-2.6-6.2

-10.40.3

-2.6-0.5-5.0-5.2

Anaerobic

nil§-0.8nil

-0.3nilnilnilnil

Autoclave

milnilnilnilnilnilnilnil

t The aerobic values represent NOj-N, whereas the anaerobic and autoclave values represent NHJ-N.t Soluble organic N in extracts prior to incubation (14 d) or autoclaving (16 h at 121 °C).

§ Below 0.1 mg N kg-' soil.

the soluble organic N values in Table 2 were obtainedafter subtacting any NH^ in the mineralization ex-tracts from the soluble TKN determinations. Other-wise, an erroneous picture may develop regardingamounts of soluble organic N present in mineraliza-tion extracts. With three of the soils, Claremore, Mus-kogee, and Ruston si, appreciable amounts of NH4(e.g., 5-12 mg N kg~' soil) were present in the min-eralization extracts.

Soluble Organic N AvailabilityThe water soluble organic N components in min-

eralization extracts are suspected to be readily suscep-tible to mineralization (Bremner, 1962; Smith et al.,1980; Stanford and Smith, 1972). However, an alter-native view may also be raised. Namely, if the solubleorganic N in the extracts is so susceptible to miner-alization, why is it not readily mineralized during thesoil incubation process? To determine N availabilityof the soluble organic N in the mineralization extractshere, the following three indexes were used: (i) aerobicN mineralization, (ii) anaerobic N mineralization, and(iii) autoclave-distillable N. In each case, the resultswere adjusted for inorganic N in the extracts prior tousing the index. Because of the small amounts of sol-uble organic N in the extracts, no attempt was madeto distinguish indigenous N from fertilizer-incorpo-rated N. Results for the indexes, summarized in Table3, show quite clearly that soluble organic N in theextracts was not exceptionally susceptible to miner-alization. This was the case irrespective of which in-dex was used. (Evidently, some original NOj in theaerobic-treated extracts was denitrified, thereby re-sulting in the fairly high negative values.) Moreover,essentially all evidence of susceptibility was observedwith the soluble organic N removed prior to start ofincubation. Such findings support the alternative viewraised above.

Organic N Forms in ExtractIn an effort to learn more about the nature of the

soluble organic N components in the mineralizationextracts, the characterization procedure of Porter etal. (1964) was employed. This procedure separates theorganic N forms into three broad fractions by hy-drolysis with HC1. Fraction 1 (distillable acid-solubleNH4) includes ammonia N released during hydrolysis,amide N, and amino sugar N. Fraction 2 (nondistill-able acid-soluble N) includes amino acid N, and Frac-

tion 3 (acid-insoluble N) includes acid-insoluble hu-min N or unhydrolyzed humin and N of the insolubleresidue. Application of the fractionation procedure tothe initial extracts indicated that Fraction 1 comprised33 ± 23% (average and standard deviation for theeight soils) of the soluble N, and the remainder wasin Fraction 2 (there was no Fraction 3 because aninsoluble N residue did not exist). This means thatabout one-third of the soluble N existed in the morelabile, distillable forms, and the remainder as aminoacids. Comparable values for soil organic N fractionsare about one-fourth and one-half, respectively (Smithand Young, 1975). Evidently, then there is nothingparticularly unique about the soluble organic N dis-tributions in the mineralization extracts.

SUMMARY AND CONCLUSIONSResults with eight agricultural soils, comprising five

soil orders, show soluble organic N losses represented,on average, about 5% or less of the inorganic N pro-duced during the mineralization process. This was thecase for both indigenous and fertilizer-incorporated Nin the soils. Highest losses of soluble organic N oc-curred in the preleaching prior to incubation, and areattributed to factors (e.g., freshly decomposed plantresidues, pretreatments, etc.) other than the mineral-ization process per se. The soluble organic N removedby leaching was not found to be exceptionally suscep-tible to mineralization, utilizing three different Navailability indexes. Moreover, distribution of solubleorganic N components in the mineralization extractsdid not differ drastically from that in soil. In general,the results presented here indicate that leaching of sol-uble organic N components does not substantiallylower subsequent N mineralization values. It shouldbe emphasized, however, that my results were ob-tained using initially air-dry soil samples and may notbe directly applicable to the field situation. Neverthe-less, I conclude that inorganic N production alone (i.e.,NOj and NH^) can provide a representative pictureof a soil's potential N supplying power.

ACKNOWLEDGMENTAppreciation is expressed to D.A. Hewes and L.B. Young

for analytical assistance.

1194 SOIL SCI. SOC. AM. J., VOL. 51, 1987