Non-Peer ReviewedOne benefit often cited for legumes crops is that they contribute N to subsequent crops, but the magnitude of this effect has been difficult to quantify. A study was conducted to compare how much and when N from fertilizer and residue sources was taken up by wheat Wheat straw (W), lentil straw (L), and lentil green manure (G), unenriched or enriched in 15N, were both surface placed and incorporated into microplots (10 x 40 cm) in the field in the fall of 1988. In the spring of 1989 wheat was planted
in all microplots, and unenriched and enriched fertilizer N was added to microplots containing enriched and unenriched plant residues, respectively. Microplots were destructively sampled at planting and at 6, 10 and 13 weeks after planting. Approximately 29 % of added fertilizer was recovered in wheat tops by 6 weeks after planting in all treatments except incorporated W, where immobilization reduced this value to 19 %. Maximum recoveries of fertilizer 15N were 34 % by the final sampling date. The proportion of residue 15N recovered in wheat tops at the final sampling dates was 19 and 11 % from incorporated and surface-placed G, respectively, and 5.4 and 5.3 % from L and W, respectively. Surface placement of residues reduced immobilization of fertilizer N but increased losses of residue N. Comparisons of N availability based on recovery of 15N may be misleading because 15N recovery does not account for changes in mineralization of native N, which is likely to be affected unequally by the addition of different N sources

Bremer, E.

van Kessel, C.

English

eCommons@USASK

Availability of N to plants from legume and fertilizer sources: which is greater?

University of Saskatchewan Research Archive

University of SaskatchewanHARVEST, https://harvest.usask.ca/College of Agriculture and Bioresources Soil Science1990-03-01Availability of N to plants from legumeand fertilizer sources: which is greater?Bremer, E.Department of Soil Science, University of Saskatchewan, Saskatoon, SKhttp://hdl.handle.net/10388/11414Downloaded from HARVEST, University of Saskatchewan's Repository for Research- 11 -1.2 Availability of N to Plants from Legume and Fertilizer Sources: IN1RODUCTION Which is Greater? E. Bremer and C. van Kessel (Project funded by Esso Chemical Canada and Natural Sciences and Engineering Rese~ch Council of Canada) Several methods are available to determine plant recovery of N from various sources. N balance methods are based on the difference in total N uptake between amended and unamended treatments. With this method the observed recoveries of applied green manure N ranged from 0 to 149%, with most values between 20 and 60%. Another method more common in field studies has been to compare N response following legumes to that following a non-legume. This type of study showed that N from legume green manures was from 16 to 92% and as effective as fertilizer N. Problems with these methods include poor precision, requirement of a N deficient soil, and responses due to factors other than N, such as changes in soil physical structure, incidence of disease or weeds, or release of growth-promoting or inhibiting substances. A direct and generally more accurate method of determining recovery of added N involves adding a N source enriched in the heavier N isotope, 15N, and then determining how much of the added 15N is taken up by a subsequent crop. This method to showed that 26.5% of N from soybean residue was taken up by the following crop in a pot study. The objectives of our study were to determine how much and when N from fertilizer, lentil green manure, lentil straw or wheat straw was taken up by a subsequent crop, and to compare surface placement and incorporation of residues on recovery of fertilizer and residue N. The 15N balance method was used to meet this objective. ~ 12 -MATERIALS ANP l\1ETHODS Production of 15N-enriched and unenriched residues was carried out in 1988. Microplots (0.5 m2 by 20 em depth) were installed in field plots containing spring-seeded Neepawa wheat and Laird lentil and late-July-seeded Indianhead lentil. Fertilizer N enriched in 15N ( 10 to 17 atom % 15N excess) was applied to microplots three or four times during the growing season; total fertilizer added was equivalent to 6.0 and 10.5 g m-2 for late-seeded and spring-seeded microplots, respectively. Similar rates of unenriched fertilizer N were applied to a square meter area in the same plot Early-seeded microplots were harvested on Aug. 29 and the late-seeded microplot was harvested on Sept. 23. Residues were air-dried. ground with a Wiley mill to pass a 2 mm screen, and then analyzed for N concentration and 15N abundance (Table 1.2.1). Table 1.2.1 N and 15N content of residues used. Green manure Lentil straw Wheat straw %N 4.05 1.29 0.94 C/N ratio 10.4 32.6 44.7 Atom % 15N excess 2.665 3.440 4.043 The experimental site was located at the Saskatchewan Irrigation Research Center at Outlook in an area in which unfertilized wheat had been grown in 1988. The soil was a Bradwell sandy loam, pH 7.8, 1.6% C, 0.16%N, mineral Nand P of 6.7 and 8.2 Jlg g-1 in the 0-10 em depth and 7.7 and 2.4Jlg g-1 in the 10-30 em depth, respectively. Microplots of PVC pipe, 10 em in diameter by 40 em in length, were pressed into the ground with hydraulic force in a grid pattern (approximately 0.4 m by 0.4 m). Soil compaction during installation of the microplots was minimal. - 13 -15N-enriched and unenriched residues of the Indianhead lentil green manure (G), Laird lentil straw (L), and Neepawa wheat straw (W) were surface-placed and incorporated (2.5 to 7.5 em) on Oct. 5, 1988. A treatment with no added residue (NR) was also included. The following spring (May 8, 1989) microplots containing 15N-enriched residue were amended with 24 mg ofunenriched NJ4+-N (ammonium sulfate; equivalent to 3.0 mg N m·2) while microplots containing unenriched residue were amended with 24 mg N of 15N-enriched NJ4+-N (9.67 atom% 15N excess). Four seeds of wheat were then planted in each microplot; after two weeks these were thinned to two wheat plants microploel. The microplots were rain-fed except for an additional5 em added between weeks 6 and 10 weeks when rainfall was negligible. Three microplots of each treatment were removed from the field at the time of planting (15N-enriched microplots only), and at 6, 10 and 13 weeks after planting, which coincided with emergence of the fifth leaf, culm elongation, and maturity, respectively. Microplots were brought back intact to Saskatoon. Wheat tops were first removed and placed in a forced-air oven at 50°C. The soil was removed in three depths: 0-10 em, 10-25 em, and 25-40 em. Soil samples were well mixed and analyzed for total and mineral N. Subsamples for total N were dried at 60°C and then ground with a mortar and pestle. Wheat tops were finely ground with a cyclone mill. Grain and straw were analyzed separately at the fmal sampling date. Total soil and wheat N was determined by micro-Kjeldahl analysis modified to include nitrate and nitrite forms. Mineral N was determined . by steam distillation with MgO and Devarda alloy of extracts obtained by shaking soil with 2M KCl for 1 hour. 15N analysis was carried out by converting NJ-4+ to N2 by LiBrOH. and determining the 15N:14N ratio with a VG Micromass 602E isotope ratio mass spectrometer. The proportion of N derived from fertilizer (Ndff) or residue (Ndfr) was calculated as atom% 15N excess ofN in wheat or soil pool divided by the atom% 15N excess of N added as fertilizer or residue. - 14 -RESULTS AND DISCUSSION Total N in the tops of wheat was not significantly affected by type or placement of residue at any of the sampling dates (Figure 1.2.1), indicating that N was probably not limiting in this experiment. Tiris was likely because during early growth added fertilizer N could likely meet N requirements while during later growth N from below the microplots could likely meet N requirements. 30 D No residue ~ Incorporated ~ rll G ..... 11 L ~-..c:~ 20 em w ~8 ·=z Surface placed zb.O ~- ~ G .... 10 e2 II L lillJ w 0 6 10 13 Weeks from planting Figure 1.2.1 N accumulation of N in wheat tops at 6, 10 and 13 weeks after planting in each treatment Error bars are standard deviations. Recoveries of fertilizer 15N in the tops of wheat was similar following G, surface-placed W, surface-placed Land NR; maximum recoveries of approximately 34% were obtained by the final sampling date (Figure 1.2.2). These recoveries are somewhat low due to losses of approximately 30% between 0 and 6 weeks (Figure 1.2.3). Within 16 days of planting 9 em of rain had been r_eceived at the experimental site, and thus losses Cl.l g. ..... 36 ... ~ 0 ..c: ~ 32 c: ·--z~ 28 ~'"CI ~-o"i ~L ..... 24 :.=o ..... t:~ 20 .,£_. ..... 0 c:- 16 ~ 12 u 0 ~ 4 - 15 -{No residue, G Surface-placed L & W Incorporated L . Incorporated W Average standard deviation 6 8 10 12 14 Weeks after planting Figure 1.2.2 Recovery of fertilizer 15N in wheat tops at 6, 10 and 13 weeks after planting. 100 z 80 II") -'"" 0 N ~:g 60 .,£"C! ..... ~ 0 ..... 40 c:.o .9~ .... ::1 .J:J 20 'E Cl.l 0 0 6 10 13 No residue. G Surface-placed L & W 6 10 13 Incorporated W Unaccounted Organic Mineral Wheat Figure 1.2.3 Distribution of fertilizer 15N at 6, 10 and 13 weeks after planting. - 16 -likely occurred through denitrification and/or leaching. Recovery was slightly reduced by incorporation of L and significantly reduced by incorporation of W due to immobilization of fertilizer lSN (Figures 1.2.2 and 1.2.3); losses of fertilizer 15N were similar to that of the other treatments (Figure 1.2.3). Reduced immobilization of fertilizer 15N due to surface placement of residues with a high C/N ratio has also been observed in other studies (Smith and Sharpley. 1990). and can be attributed to reduced microbial growth and to spatial separation of fertilizer 15N from the decomposing residue. Recoveries of residue l5N in the tops of wheat were always lower than that of fertilizer 15N: a maximum recovery of 19% was obtained from incorporated G at the final sampling date (Figure 1.2.4). Less residue 15N was recovered from surface-placed G than from incorporated G because of greater losses of 15N from surface-placed G (Figures 1.2.4 and 1.2.5). Losses of 15N from surface-placed G may be due to volatilization of N <1'.! 9' 24 .... c:d ~ .c 20 ~ c .... zS 16 ll"l~ -"'0 ~~ 12 ::l..._. :'90 Surface-placed G Cl'.l~ ~- 8 ...... 0 L,W "'"' 4 a) E; 0 ~ ~ 4 6 8 10 12 14 Weeks after planting Figure 1.2.4 Recovery of residue 15N in wheat tops at 6. 10 and 13 weeks after planting. 100 z 11"'1 - 80 0 ~-:91S cn-e ~"e 60 ._~:U 0'-c:: 0 0~ 40 $= .._, ~ ..0 'E 20 en ..... Q 0 0 6 10 13 Incorporated G - 17 -0 6 10 13 Swface-placed G Organic Mineral Wheat Figure 1.2.5 Distribution of residue 15N in G treatments at 0, 6, 10 and 13 weeks after planting. during decomposition or to leaching during spring runoff. Increased NH3 evolution after alfalfa was cut has been observed and losses of green manure N due to volatilization of approximately 20% have been observed. The data suggests that losses from incorporated G were most likely due to leaching: losses occurred between 0 and 6 weeks after planting, 15% of added 15N was present in mineral form in the 10-40 em depths at the time of planting, and part of the 15N lost by 6 weeks after planting was recovered by wheat plants after 6 weeks. Recovery of residue 15N from surface-placed and incorporated L and W was similar; a maximum recovery of 5.8% in wheat tops was obtained for these residues. Surface placement of W and L did not affect recovery of 15N in wheat but increased unaccounted 15N from 0 to 10%, again most likely due to volatilization of 15N. The low recovery of 15N from these residue can be attributed to their wide C:N ratio:microorganisms breaking down these residues would require any N present for their own growth (Figure 1 2.6). 100 z ll'l - 80 0 =-:9] 00"'0 60 2:!"'0 l;..o~ O~;..o c:: 0 40 0~ ·::~ ::::~-,J:j 'E 20 00 ·-Q 0 0 6 10 13 Incorporated L & W - 18 -Unaccounted 0 6 10 13 Surface-placed L & W Organic Mineral Wheat Figure 1.2.6 Distribution of residue 15N inLand W treatments at 0, 6, 10 and 13 weeks after planting. Although only 20% of N in wheat tops was accumulated by 6 weeks, close to 85% of fertilizer 15N and approximately 60% of residue 15N was accumulated by this time. The main reason for poor synchronization between N and 15N uptake is that N from both fertilizer or residues is most readily available to soil microoganisms initially and quickly becomes stabilized in more resistant forms. The recoveries of residue 15N from G were similar to those obtained from other studies using 15N balance for residues with a high N concentration (Table 1.2.2). In general, less N from plant residues was accumulated by a growing crop than was or could be expected from fertilizer. However, 15N recoveries may underestimate the total amount of N released. especially with added residues, because it does not account for increased net mineralization of native N. Higher but often variable estimates of N recovery based on differences in N uptake or N response may often be real and due to the interaction of added residue with native soil N. - 19 -Table 1.2.2 Reported recoveries by a subsequent crop of 15N from 15N-enriched legume residues. Authors Country Residue type %Recovered Bremer and Van Kessel Canada Lentil 11, 19 Ladd et al. (1981. 1983, 1986) Australia Alfalfa 11-25 Muller and Sundman (1988) Finland Field Bean, Clover 18-24 Harris and Hesterman (1990) U.S.A. Alfalfa 17-25 CONCLUSIONS (1) Maximum recoveries of fertilizer 15N were 34% by the fmal sampling date. (2) The proportion of residue 15N recovered in wheat tops at the final sampling dates was 19% and 11% from incorporated and surface-placed G, respectively, and 5.4 and 5.3% from L and W, respectively. (3) Surface placement of residues reduced immobilization of fertilizer N but increased losses of residue N. (4) Comparisons ofN availability based on recovery of 15N may be misleading because 15N recovery does not account for changes in mineralization of native N, which is likely to be affected unequally by the addition of different N sources. 

Availability of N to Plants- from Legume and Fertilizer Sources: Which is Greater? Eric Bremer and Chris van Kessel Department of Soil Science University of Saskatchewan Abstract One benefit often cited for legumes crops is that they contribute N to subsequent crops, but the magnitude of this effect has been difficult to quantify. A study was conducted to compare how much and when N from fertilizer and residue sources was taken up by wheat Wheat straw (W), lentil straw (L), and lentil green manure (G), unenriched or enriched in 15N, were both surface placed and incorporated into microplots (10 em diameter x 40 em) in the field in the fall of 1988. In the spring of 1989 wheat was planted in all microplots, and unenriched and enriched fertilizer N was added to microplots containing enriched and unenriched plant residues, respectively. Microplots were destructively sampled at planting and at 6, 10 and 13 weeks after planting. Approximately 29% of added fertilizer was recovered in wheat tops by 6 weeks after planting in all treatments except incorporated W, where immobilization reduced this value to 19%. Maximum recoveries of fertilizer 15N were 34% by the final sampling date. The proportion of residue 15N recovered in wheat tops at the fmal sampling dates was 19% and 11% from incorporated and surface-placed G, respectively, and 5.4 and 5.3% from L and W, respectively. Surface placement of residues reduced immobilization of fertilizer N but increased losses of residue N. Comparisons of N availability based on recovery of 15N may be misleading because 15N recovery does not account for changes in mineralization of native N, which is likely to be affected unequally by the addition of different N sources. Introduction Several methods are available to determine plant recovery of N from various sources. N balance methods are based on the difference in total N uptake between amended and unamended treatments. With this method Lohnis (1926) observed recoveries of applied green manure N from 0 to 149%, with most values between 20 and 60%. Another method more common in field studies has been to compare N response following legumes to that following a non-legume. Stickler et al. (1959) concluded from this type of study that N from legume green manures was from 16 to 92% as effective as fertilizer N. Problems with these methods include poor precision, requirement of a N deficient soil, and responses due to factors other than N, such as changes in soil physical structure, incidence of disease or weeds, or release of growth-promoting or inhibiting substances. A direct and generally more accurate method of determining recovery of added N involves adding a N source enriched in the heavier N isotope, 15N, and then determining how much of the added 15N is taken up by a subsequent crop. Norman and Werkman (1943) used this method to show that 26.5 % of N from soybean residue was taken up by the following crop in a pot study. The objectives of our study were to determine how much and when N from fertilizer, lentil green manure, lentil straw or wheat straw was taken up by a subsequent crop, and to compare surface placement and incorporation of residues on recovery of fertilizer and residue N. The 15N balance method was used to meet this objective. 262 Materials and Methods Production of 15N-enriched and unenriched residues was carried out in 1988. Microplots (0.5 m2 by 20 em depth) were installed in field plots containing spring-seeded Neepawa wheat and Laird lentil and late-July-seeded Indianhead lentil. Fertilizer N enriched in 15N (10 to 17 atom % 15N excess) was applied to microplots three or four times during the growing season; total fertilizer added was equivalent to 6.0 and 10.5 g m-2 for late-seeded and spring-seeded microplots, respectively. Similar rates of unenriched fertilizer N were applied to a square meter area in the same plot. Early-seeded micoplots were harvested on Aug. 29 and the late-seeded micropot was harvested on Sept 23. Residues were air dried, ground with a Wiley mill to pass a 2 mm screen, and then analysed for N concentration and 15N abundance (Table 1). Table 1 N and 15N content of residues used %N C/Nratio Atom % 15N excess Green manure Lentil straw 4.05 10.4 2.665 1.29 32.6 3.440 Wheat straw 0.94 44.7 4.043 The experimental site was located at the Saskatchewan Irrigation Research Center at Outlook in an area in which unfertilized wheat had been grown in 1988. The soil was a Bradwell sandy loam, pH 7.8, 1.6% C, 0.16%N, mineral Nand P of 6.7 and 8.2 J.Lg g~l in the 0-10 em depth and 7.7 and 2.4 J.Lg g-1 in the 10-30 em depth, respectively. Microplots of PVC pipe, 10 em in diameter by 40 em in length, were pressed into the ground with hydraulic force in a grid pattern (approximately 0.4 m by 0.4 m). Soil compaction during installation of the microplots was minimal. 15N-enriched and unenriched residues of the Indianhead lentil green manure (G), Laird lentil straw (L), and Neepawa wheat straw (W) were surface-placed and incorporated (2.5 to 7.5 em) on Oct. 5, 1988. A treatment with no added residue (NR) was also included. The following spring (May 8, 1989) microplots containing 15N-enriched residue were amended with 24 mg of unenriched NH4+-N (ammonium sulfate; equivalent to 3.0 mg N m-2) while microplots containing unenriched residue were amended with 24 mg N of 15N-enriched NH4+-N (9.67 atom % 15N excess). Four seeds of wheat were then planted in each microplot; after two weeks these were thinned to two wheat plants microplot·l. The microplots were rain-fed except for an additiona15 em added between weeks 6 and 10 weeks when rainfall was neglible. Three microplots of each treatment were removed from the field at the time of planting (15N-enriched microplots only), and at 6, 10 and 13 weeks after planting, which coincided with emergence of the fifth leaf, culm elongation, and maturity, respectively. Microplots were brought back intact to Saskatoon. Wheat tops were first removed and placed in a forced-air oven at 50 °C. The soil was removed in three depths: 0-10 em, 10-25 em, and 25-40 em. Soil samples were well mixed and analysed for total and mineral N. Subsamples for total N were dried at 60°C and then ground with a mortar and pestle. Wheat tops were fmely ground with a cyclone mill. Grain and straw were analysed separately at the final sampling date. Total soil and wheat N was determined by micro-Kjeldahl analysis modified to include nitrate and 263 nitrite forms (Bremner and Mulvaney, 1982). Mineral N was determ1ned by steam distillation with MgO and Devarda alloy of extracts obtained by shaking soil with 2 M KCl for 1 hour. 15N analysis was carried out by converting NJ4+ to Nz by LiBrOH (Ross and Martin, 1970) and determining the 15N:14N ratio witha V'GMicromass 602E isotope ratio mass spectrometer. The proportion ofN derived from fertilizer (Ndff) or residue (Ndfr) was calculated as atom %15N excess ofN in wheat or soil pool divided by the atom %15N excess ofN added as fertilizer or residue. Results and Discussion Total N in the tops of wheat was not significantly affected by type or placement of residue at any of the sampling dates (Fig. 1), indicating that N was probably not limiting in this experiment. This was likely because during early growth added fertilizer N could likely meet N requirements while during later growth N from below the microplots could likely meet N requirements. 30 0 No residue en Incorporated g. II G .... ... 111 L i'i-..r:::.~ 20 11 w ~E c: ·-z Surface placed zt:)Q -a- l?1l G .... 10 ~ • L [21 w 0 6 10 13 Weeks from planting Fig. 1 N accumulation ofN in wheat tops at 6, 10 and 13 weeks after planting in each treatment. Error bars are standard deviations. Recoveries of fertilizer 15N in the tops of wheat was similar following G, surface-placed W, surface-placed Land NR; maximum recoveries of approximately 34% were obtained by the final sampling date (Fig. 2). These recoveries are somewhat low due to losses of approximately 30% between 0 and 6 weeks (Fig. 3). Within 16 days of planting 9 em of rain had been received at the experimental site, and thus losses likely occurred through denitrification and/or leaching. Recovery was slightly reduced by incorporation of L and significantly reduced by incorporation of W due to immobilization of fertilizer 15N (Figs. 2 and 3); losses of fertilizer 15N were similar to that of the other treatments (Fig. 3). Reduced immobilization of fertilizer 15N due to surface placement of residues with a high C/N ratio has also been observed in other studies (Smith and Sharpley, 1990), and can be attributed to reduced microbial growth and to spatial separation of fertilizer 15N from the decomposing residue. 264 v.l §-.... ..... ~ ~ ~ c:: .... z::a' lliQ) -:g ts~ Nc.,. ;.:=o ·~~ ~-~ ~ ~ 0 ~ 36 32 28 24 20 16 12 4 { No residue, G Surface-placed L & W Incorporated L Incorporated W Average standard deviation 6 8 10 12 14 Weeks after planting Fig. 2 Recovery of fertilizer 15N in wheat tops at 6, 10 and 13 weeks after planting. 0 6 10 13 No residue, G Surface-placed L & W 6 10 13 Incorporated W Fig. 3 Distribution of fertilizer 15N at 6, 10 and 13 weeks after planting. Unaccounted Organic Mineral Wheat Recoveries of residue 15N in the tops of wheat were always lower than that of fertilizer 15N: a maximum recovery of 19% was obtained from incorporated G at the final sampling date (Fig. 4). Less residue 15N was recovered from surface-placed G than from incorporated G because of greater losses of 15N from surface-placed G (Figs. 4 and 5). Losses of 15N from surface-placed G may be due to volatilization of N during decomposition or to leaching during spring runoff. Dabney and Bouldin (1985) measured increased NH3 evolution after-alfalfa was cut and Janzen et al. (1990) have also observed losses of green manure N due to volatilization of 265 approximately 20%. The data suggests that losses from incorporated G were most likely due to leaching: losses occurred between 0 and 6 weeks after planting, 15% of added 15N was present in mineral form in the 10-40 em depths at the time of planting, and part of the 15N lost by 6 weeks after planting was recovered by wheat pliirtts after 6 weeks. Recovery of residue 15N from surface-placed and incorporated L and W was similar; a maximum recovery of 5.8 % in wheat tops was obtained for these residues. Surface placement of W and L did not affect recovery of 15N in wheat but increased unaccounted 15N from 0 to 10%, again most likely due to volatilization of 15N. The low recovery of 15N from these residue can be attributed to their wide C:N ratio: microorganisms breaking down these residues would require any N present for their own growth (Bartholemew, 1965). rll 9" 24 ~ i 20 8 4 L---.... · Incorporated G Surface-placed G L,W o+-~--~~--r--r~--,-~--~~ 4 6 8 10 12 14 Weeks after planting Fig. 4 Recovery of residue 15N in wheat tops at 6, 10 and 13 weeks after planting. 100 z ll"l - 80 G) =-:sl''B e:g 60 co'(; = 0~ 40 ·=-_g ·a 20 rll ·-Q 0 0 6 10 13 Incorporated G 0 6 10 13 Surface-placed G Organic Mineral Wheat Fig. 5 Distribution of residue 15N in G treatments at 0, 6, 10 and 13 weeks after planting. 266 z II") -Q,) ::s_ :9'3 Vl"' ~"0 '+-ii:U 0'+-4 c::O 0~ ·:::: ::s'-' .c ·a VI ..... 0 100 80 60 40 20 0 0 6 10 13 0 6 10 13 Unaccounted Organic Mineral Wheat Incorporated L & W Surface-placed L & W Fig. 5 Distribution of residue 15N in L and W treatments at 0, 6, 10 and 13 weeks after planting. Although only 20% of N in wheat tops was accumulated by 6 weeks, close to 85% of fertilizer 15N and approximately 60% of residue 15N was accumulated by this time. The main reason for poor synchronization between N and 15N uptake is that N from both fertilizer or residues is most readily available to soil microoganisms initially and quickly becomes stabilized in more resistant forms (Bartholemew, 1965; Jansson and Persson, 1982). The recoveries of residue 15N from G were similar to those obtained from other studies using 15N balance for residues with a high N concentration (Table 2). In general, less N from plant residues was accumulated by a growing crop than was or could be expected from fertilizer. However, 15N recoveries may underestimate the total amount ofN released, especially with added residues, because it does not account for increased net mineralization of native N (Jansson and Persson, 1982). For example, Lohnis (1926) obtained recoveries based on differences in total N accumulation as high as 200%, indicating a significant increase in mineralization of native N. Thus, the higher but often variable estimates of N recovery based on differences in N uptake or N response may often be real and due to the interaction of added residue with native soil N. Table 2 Reported recoveries by a subsequent crop of 15N from 15N-enriched legume residues. Authors Country Residue type %Recovered Bremer and Van Kessel Canada Lentil 11, 19 Ladd et al. (1981, 1983, 1986) Australia Alfalfa 11-25 Muller and Sundman (1988) Finland Field Bean, Oover 18-24 Harris andHesterman (1990) U.S.A. Alfalfa 17-25 267 References Bartholemew, W.V. 1965. Mineral~atiol} and immobilization of nitrogen in the decomposition of plant and animal residues. pp~ 285;.306 In W.V. Bartholemew and F.E. Clark (eds.) Soil Nitrogen. Amer. Soc. Agron., Madison. Bremner J.M. and C.S. Mulvaney. 1982. Nitrogen- Total. pp. 595-624 In A.L. Page, R.H. Miller and D.R. Keeney (eds.) Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties, 2nd edition . American Society of Agronomy, Madison. Dabney, S.M. and D.R. Bouldin. 1985. Fluxes of ammonia over an alfalfa field. Agron. J. 77: 572-578. Harris, G.H. and O.B. Hesterman. 1990. Quantifying the nitrogen contribution from alfalfa to soil and two succeeding crops using nitrogen-15. Agron. J. 82: 129-134. Jansson, S.L. and J. Persson. 1982. Mineralization and immobilization of soil nitrogen. pp. 229-252 In F.J. Stevenson (ed.) Nitrogen in Agricultural Soils. Amer. Soc. Agron., Madison. Janzen, H.H., V.O. Biederbeck, and S.M. McGinn. 1990. Volatile N loss from decomposing green manure. Soils and Crops Workshop, Feb. 22-23, Univ. of Sask., Saskatoon. Ladd, J.N., J.M. Oades, and M. Amato. 1981. Distribution and recovery of nitrogen from legume residues decomposing in soils sown to wheat in the field. Soil Biol. Biochem. 13: 251-256. Ladd, J.N., M. Amato, R.B. Jackson and J.H.A. Jackson. 1983. Utilization by wheat crops of nitrogen from legume residues decomposing in soils in the field. Soil Bioi. Biochem. 15: 231-238. Ladd, J .N. and M. Amato. 1986. The fate of nitrogen from legume and fertilizer sources in soils successively cropped with wheat under field conditions. Soil Biol. Biochem. 18: 417-425. Lohnis, F. 1926. Nitrogen availability of green manures. Soil Sci. 22: 253-290. Norman, A. G. and C.H. Werkman. 1943. The use of the nitogen isotope Nl5 in determining nitrogen recovery from plant materials decomposing in soil. J. of the Amer. Soc. Agron. 25: 1023-1025. Muller, M.M. and V. Sundman. 1988. The fate of nitrogen (15N) released frQm different plant materials during decomposition under field conditions. Plant Soil105: 133-139. Ross P.J. and A.E. Martin. 1970. A rapid procedure for preparing gas samples for N-15 determinations. Analyst 95: 817-822. Smith, S.J. and A.N. Sharpley. 1990. Soil nitrogen mineralization in the presence of surface and incorporated residues. Agron. J. 82:112-116. Stickler, F.C., W.D. Shrader, and I.J. Johnson. 1959. Comparative value of legume and fertilizer nitrogen for com production. Agron. J. 51: 157-160. 268 

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Availability of N to plants from legume and fertilizer sources: which is greater?

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